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Geology and Mineralogy.

RECENT PROGRESS OF GEOLOGY.

THE President of the British Association, in his inaugural Address, at their recent Annual Meeting, observed :-The science of Geology appears, of late years, to have entered upon a new phase of its development,- -one characterized by a stricter reference of its speculative views to the principles of those sciences with which it is connected, and upon which it ought to be based. We have an example of this in the able Memoirs of Mr. Hopkins, on what may be called Dynamical Geology, including the changes which have taken place in the earth's crust by the operation of internal forces. Another instance of the application of sound physical principles to this science is found in the explanations which have been recently offered of the phenomena of slaty cleavage. A Report on this interesting subject was presented to the Association by Professor Phillips at its last Meeting, and will be found in the volume just published. These sounder views originate, I believe, with himself and with Mr. Sharpe; but they have been enlarged and confirmed by Mr. Sorby, Dr. Tyndall, and Professor Haughton.

We have another interesting proof of the readiness of geologists of the present day to submit their views to the test of exact observation, in the measurements undertaken by Mr. Horner for the purpose of approximating to the age of sedimentary deposits. Of the geological changes still in operation, none is more remarkable than the formation of deltas at the mouths of great rivers, and of alluvial land by their overflow. Of changes of the latter kind, perhaps the most remarkable is the great alluvial deposit formed in the valley of the Nile by the annual inundations of that river; and here it fortunately happens that history comes to the aid of the geologist. These sedimentary deposits have accumulated round the bases of monuments of known age; and we are, therefore, at once furnished with a chronometric scale by which the rate of their formation may be measured.

The first of the series of measurements undertaken by Mr. Horner was made with the co-operation of the Egyptian Government, around the obelisk of Heliopolis, a monument built, according to Lepsius, 2300 years B.C. A more extensive series of researches has been since undertaken in the district of Memphis; but Mr. Horner has not yet, I believe, published the results. The problems now to be solved in Paleontology are clearly defined in the enunciation of the problem recently proposed by the French Academy of Sciences as one of its prize questions-viz., "to study the laws of distribution of organic beings in the different sedimentary rocks, according to the order of their superposition; to discuss the question of their appearance or disappearance, whether simultaneous or successive; and to determine the nature of the relations which subsist between the existing organic kingdom and its anterior states." The prize was obtained by Professor Bronn, of Heidelberg; and his

Memoir, of which I have only seen an outline, appears to be characterized by views at once sound and comprehensive. The leading result seems to be, that the genera and species of plants and animals, which geology proves to have existed successively on our globe, were created in succession, in adaptation to the existing state of their abode, and not transmuted, or modified, as the theory of Lamarck supposes, by the physical influences which surrounded them.

THE SEA LEVEL AND GEOLOGICAL EPOCHS.

A PAPER has been read to the British Association "On the Existence of Forces capable of changing the Sea-level during different Geological Epochs," by Professor Hennessy. If, in assuming its present state from an anterior condition of entire fluidity, the matter composing the crust of the earth underwent no change of volume, the direction of gravity at the earth's surface would remain unchanged, and consequently the general figure of the liquid coating of our planet. If, on the contrary, as we have reason to believe, a change of volume should accompany the change of state of the materials of the earth from fluidity to solidity, the mean depth of the ocean would undergo gradual, though small, changes over its entire extent at successive geological epochs. This result is easily deduced from the general views contained in other writings of the author, whence it appears, that if the surface stratum of the internal fluid nucleus of the earth should contract when passing to the solid state, a tendency would exist to increase the ellipticity of the liquid covering of the outward surface of the crust. A very small change of ellipticity would suffice to lay bare or submerge extensive tracts of the globe. If, for example, the mean ellipticity of the ocean increased from 3 to 5, the level of the sea would be raised at the equator by about 228 feet, while under the parallel of 52° it would be depressed by 196 feet. Shallow seas and banks in the latitudes of the British isles, and between them and the pole, would thus be converted into dry land, while low-lying plains and islands near the equator would be submerged. If similar phenomena occurred during early periods of geological history, they would mauifestly influence the distribution of land and water during these periods, and with such a direction of the forces as that referred to, they would tend to increase the proportion of land in the polar and temperate regions of the earth, as compared with the equatorial regions during successive geological epochs. Such maps as those published by Sir Charles Lyell on the distribution of land and water in Europe during the tertiary period, and those of M. Elie de Beaumont, contained in Beaudant's Geology, would, if sufficiently extended, assist in verifying or disproving these views.

GRANITITE AND GRANITE.

GUSTAV ROSE has made to the German Association some observations on the gneiss which forms the north-western limit of the Granitite of the Riesengebirge, and of the Granite which occurs in it; he also

spoke of the relation of granite to gneiss in general. The boundaries betwixt the two could, he said, be very distinctly drawn in the Riesengebirge. In 1856, at Vienna, the learned Professor gave an account of some recent investigations which he had made in the Riesengebirge and Isergebirge, with a view to determine the exact limits betwixt granitite and granite; and assigned the reasons which had induced him to regard the former as a separate species of rock from the latter. These reasons were-first, the distinct mineral composition-the white mica of the granite being entirely wanting; secondly, the accurate limits which can be drawn betwixt it and the granite of the Isergebirge; and, thirdly, the circumstance that mixtures of similar composition to the granitite of the Riesengebirge and Isergebirge occurred in the most diverse localities. From the relations of the granitite to the granite, the Professor considered that the former must have penetrated to the surface more recently than the latter. [See also a contribution by Rose," Uber die zur granitgruppe gehörigen Gebirgsarten," in the first volume of the Zeitschrift der Deutsch-geologischen Gesellschaft.]

FORMATION OF ROCK BASINS.

MR. J. CLEGHORN, in a paper communicated to the Geological Society by Sir R. I. Murchison, refers to the existence of Pot-holes on the shores of Caithness, which he has reason to believe were formed in the first place by the grinding action of a loose mass of rock rotated on one spot by the action of the waves, and in the second place by the wearing action of pebbles and boulders washed about within the pots or basins by the sea. Mr. Cleghorn referred also to the rock basins of Dartmoor, and the Giants' Pots of Sweden, and stated his belief that these also had originated in some modifications of a similar agency, in opposition to the opinions of other observers, who had referred the formation of some to the decomposing action of the atmosphere on granite (aided in some instances by human agency), of others to the effect of glaciers, &c.

CONDUCTING POWER OF ROCKS-MOUNTAINS NOT IMMOVABLE.

MR. HOPKINS, of Cambridge, has lately made some interesting experiments on the conductivity or conducting power of different substances for heat, of which an account was laid before the Royal Society of London in June last. Without attempting to describe his processes, we give his more important conclusions. These results are in decimals, the conductivity of "igneous rock" (trap or granite, we presume) saturated with moisture being taken for unity :

The conductivity of the following rocks is given in two statesdry, and saturated with water :—

The effect of pressure on the conducting power of substances was also tried, and proved to be almost nothing. A pressure of 7500 lb. on a square inch of beeswax, spermaceti, and chalk, had no appreciable effect. Uncompressed clay, which had a conducting power of 26, had the same raised to 33 by a pressure of 7500 lb.

Sandstone, with conducting power of 5, divided into strata each 1 foot thick, when compared with a similar mass in one block, had its conducting power diminished 1-20th. When the strata were only 6 inches thick, the diminution was 1-10th. The effect of discontinuity of substance is therefore small. Saturation with moisture, on the other hand, produces generally a great effect, as will be seen on comparing the dry and saturated blocks of chalk, the dry and saturated New Red Sandstone, and again the dry and saturated "igneous rocks."

These facts have a certain bearing on a geological question - namely, the transmission of heat from the interior of the earth to the crust. The oolite, for instance, conducts heat much better than the chalk, the sandstone better than the oolite, the igneous rock better than the sandstone, and, in all cases, the rock charged with moisture better than the dry rock. But Mr. Hopkins would have added to the value of his paper if he had ascertained by experiment the quantity of water absorbed by each rock at given temperatures, and whether the conductivity is exactly in proportion to the absorption.

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In illustration of the use that may be made of the tables, we would refer to certain remarks made by Dr. Robinson on a paper read by Professor Hennessy at the recent meeting of the British Association. The subject was "the direction of gravity at the earth's surface." In alluding to certain supposed local and temporary changes of level, he mentioned the following curious fact:"He found the entire mass of rock and hill on which the Armagh Observatory is erected to be slightly, but to an astronomer quite perceptibly, tilted or canted at one season to the east, at another to the west. This he at first attributed to the varying power of the sun's radiation to heat and expand the rock throughout the year; but he subsequently had reason to attribute it rather to the infiltration of water to the parts where the clay-slate and limestone rocks met. The varying quantity of this (water) through the year he now believed exerted a powerful hydrostatic energy by which the position of the rock is slightly varied." With the light furnished by Mr. Hopkins's experiments, we may pronounce the explanation satisfactory. Armagh and its Observatory stand at the junction of the

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mountain limestone with the clay-slate, having, as it were, one leg on the former, and the other on the latter, and both rocks probably reach downwards one or two thousand feet. When rain falls, the one will absorb more water than the other; both will gain an increase of conductive power, but the one which has absorbed most water will have the greatest increase; and being thus the better conductor, will draw a greater portion of heat from the hot nucleus below to the surface-will become, in fact, temporarily hotter, and, as a consequence, expand more than the other. In a word, both rocks will expand at the wet season; but the best conductor, or most absorbent rock, will expand most, and seem to tilt the hill to one side; at the dry season it will subside most, and the hill will seem to be tilted in the opposite direction.

The fact is curious, and not less so are the results deducible from it. First, hills are higher at one season than another, a fact we might have supposed, but never could have ascertained by measurement. Secondly, they are highest, not, as we would have supposed, at the hottest season, but at the wettest. Thirdly, it is from the different rates of expansion of different rocks that this has been discovered; had the limestone and clay-slate expanded equably, or had Armagh Observatory stood on a hill of homogeneous rock, it would have remained unknown. Fourthly, though the phenomenon is in the strictest sense earthly, it is by converse with the heavens that it has been made known to us. A variation of probably half a second, or less, in the right ascension of three or four stars, observed at different seasons, no doubt revealed the fact to the sagacious astronomer of Armagh, and even enabled him to divine its cause; which has been confirmed as the true cause, and placed in a clearer light, by the experiments of Mr. Hopkins. One useful lesson may be learned from the discovery-to be careful to erect Observatories on a homogenous foundation.—Mr. Maclaren, in the Scotsman, Oct. 10, 1857.

CRAG SHELLS AND THE CLIMATE OF ENGLAND.

SIR CHARLES LYELL, in the Supplement to the fifth edition of his Manual of Elementary Geology, deduces an important conclusion from Mr. Searles Wood's newly completed Monograph on the Crag Shells, which affords "clear evidence of a gradual refrigeration in the climate of England from the time of the older to that of the most modern Pliocene strata." On the shores of Norfolk and Suffolk there are three sandy deposits called " Crags," abounding in shells. On comparing these three Crags, the northern species of mollusca (still living in the Arctic seas) are found to increase as we ascend from the older deposits to the newer; on the other hand, the southern species yet living in the Mediterranean are found to increase as we descend from the newer deposits to the older. Contemporary with these deposits, or perhaps later, are the Hippopotamus Major, a monkey, and remains of Rhinoceros Leptorinus, found on the north bank of the Thames, in Essex; and above the whole is a sheet of ochreous gravel spread over the valley of the Thames from