6. Cambrian.-Limestone from Craigmuir near Inverary. Metamorphic and not fossiliferous; specimen from a series of alternating beds of from 6 inches to several feet thickness of limestone with clay-shales, and resting upon clay-slates. They have an E. and W. strike, with dip of 30° to N., and are continuous for many miles.

This limestone is beautifully foliated by plates of silvery mica, which separates in large quantities when the limestone is dissolved in acids. The foliation appears to be perfectly parallel to the plane of bedding of the rocks.

The limestone is compact, but not hard, and is of a grayish colour, splitting easily along the planes of foliation, and the surfaces thus exposed being covered by a beautiful white mica.

The specific gravity was found to be 2-72 at 60° F. 29.22 grs. were analysed and yielded 9.41 carbonic acid, 7.08 insoluble, residue chiefly mica, 0·06 organic matter, 0.47 sesquioxide of iron and alumina, 0.20 gr. phosphate of magnesia from the phosphoric acid, and 0.05 ditto from the magnesia determination.

These results afford the following per-centage composition:

7. White limestone forming veins and irregular masses in the hornblende schist at Kragerod, Norway, and having a peculiar shell-like laminar structure.

The hornblende schist in which this limestone occurs contains numerous veins of phosphate of lime; and this limestone was analysed under the supposition, that, as it had apparently been formed by the deposition from water filtering through the surrounding strata, it would contain a large amount of phosphoric acid, which, however, was not found to be the case. The specific gravity was found to be 2-69 at 60° F.

On analysis, 46.94 grs. yielded 4.24 insoluble, 0.36 organic matter, 0.13 alumina, with phosphoric acid and trace of iron, 41.89 carbonate of lime, 0-07 oxide of manganese, and 0.16 gr. phosphate of magnesia from the phosphoric acid determination.

54.73 grs. examined for magnesia afforded 0.14 gr. phosphate of magnesia, and 50·17 grs., dried at 300° F., yielded 0·11 water as loss.

Phil. Mag. S. 4. Vol. 13. No. 87. May 1857.

2 C

For the sake of comparison it may now be of interest to tabulate. the whole of the results of the preceding analyses.

rary.

Ysputty Rhiwlas. Dinover, Long- Inve- KraDudley. Evan. Llandeilo. mynd. geroe. 90-09 39-54 19:51 79.97 63-10 73-34 89-24 1.26 1.85 1.04 0.52 0.80 0.28 0.19

100-00 100.00 100.00 100.28 100-27 100'00 100.00

In comparing the above limestones, in respect to their chemical composition, with those of the later formations, there seems to be but little difference, if we except that the older limestones are generally more impure and, considered as rock masses, seldom contain so large an amount of carbonate of lime as the limestones of the later formations.

With regard to their respective amounts of phosphoric acid, the now fossiliferous limestones from Church Stretton and Inverary, as well as the Llandeilo limestone, contain fully as much phosphoric acid as the subsequent limestones; and if we take into consideration the respective amounts of carbonate of lime, the same may be said of the Bala limestones.

That the phosphoric acid present in these limestones has been derived from organic remains seems the more probable, from our not finding phosphoric acid in those varieties of carbonate of lime crystallized from solution; and amongst all the numerous analyses of calc-spar published, I do not find phosphoric acid mentioned as occurring; in order, however, to satisfy myself that

this element had not been overlooked by chemists, I examined carefully specimens of Iceland spar, arragonite, and calc-spar from Derbyshire and Durham, by the same method as that employed in the above analyses of the limestones, but in no case did I find any trace of phosphoric acid.

In the specimen of limestone from Krageroe, which, from its occurrence, seems probably to have been deposited from water holding lime in solution, the amount of phosphoric acid present is accounted for by the immediately surrounding rocks containing phosphate of lime; but even in this the amount is less than might be expected, and much below the per-centage in the stratified limestones.

In a single specimen of calc-spar from Naeskül, near Arendal, did I find a trace of phosphoric acid, but too small to be easily determined; in this case, however, the calc-spar lined fissures in granite which contained crystallized apatite.

The calc-sinter deposited from the Carlsbad springs was found by Berzelius to contain a small amount of phosphoric acid, but as these springs break through granite, it is possible that the phosphoric acid present in the sinter may arise from a similar

cause.

The results of these analyses, therefore, seem if anything to strengthen the view that all stratified limestones are the result of the development of organic life. It must, however, be remembered that under the term stratified limestone, are not included the irregular deposits of travertine, calc-sinter, &c., which obviously are thrown down from springs surcharged with carbonate of lime.

That beds of limestone become less and less numerous as we descend in the geological scale, is easily accounted for, even without taking into consideration the comparatively small development of organic life during those early epochs. Whatever view be taken of the origin of limestones, it cannot but be admitted that the lime which constituted them must have been originally in a state of solution, and chemical reasons may be adduced for assuming that in the earlier geological periods a but scanty supply of lime in a soluble state was present.

The oldest granitic rocks contained but a very small amount of lime, and even that amount was in a state of chemical combination not calculated for easy decomposition, consequently the debris of such rocks afforded but little lime in a soluble state.

When, however, the subsequent and more basic class of rocks made their appearance as traps, porphyries, diorites, some syenites, &c., we find a supply of soluble lime provided for the development of life and limestones. These rocks not only contained much more lime than the granites, but were themselves much

more easily decomposable compounds, and admirably fitted for yielding up lime to the action of water and carbonic acid.

Under the impression that the occurrence of magnesia in limestones first commenced during the Devonian period, Kjerulf has recently made use of this view as a means for determining the position of stata, which, not being fossiliferous, do not afford paleontological evidence of their geological position. The desirability of such a means of classification is obvious, but its correctness must be questioned.

The analyses here brought forward certainly show but very little magnesia; but I have found that two limestones imbedded in the metamorphic schists west of Krageroe, in Norway, are true dolomites, and from their position must be Cambrian. Sheerer has also noticed the occurrence of much carbonate of magnesia in the metamorphic rocks near Snarum in Norway; and dolomites occur in the Silurian rocks of Livonia, Esthland and Kurland; and I have reason for supposing that some of the rocks which, from the occurrence of dolomites in them, are classed by Kjerulf as Devonian, may possibly prove to be Cambrian.

We must also remember that the presence of magnesia is not necessarily connected with the magnesian limestone formation; as we know that mountain limestone in some parts of Leicestershire, Derbyshire, Yorkshire and Northumberland is highly magnesian; on the other hand also, many beds of the magnesian limestone series do not contain more than a trace of magnesia, although in the midst of dolomites.

The two following analyses, by Liebe, of German limestones prove this:

Thieschütz.

Schwara.

Both of these were alternating with true dolomites.

On the whole, the evidence brought forward with reference to the occurrence of magnesia in limestones seems to favour the view that its presence is due to subsequent alteration, probably by infiltration, as may be proved in many cases. If so, we may expect magnesian limestones to be of all ages.

How far organic life is capable of assimilating magnesia in place of lime is not sufficiently investigated, but the presumption

is, that an excess of magnesia is hurtful to vegetable life, and we find but little magnesia present either in fossil or recent organisms.

We know, however, that some plants, in which potash is one of the principal inorganic constituents, may, when potash fails, and soda is present or supplied as a manure, take up that alkali which otherwise is not assimilated by them to any degree; in like manner it may be possible that corals living in water containing both lime and magnesia may, after having exhausted all the lime, supply the deficiency by magnesia.

LII. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from p. 293.]

June 19, 1856.-The Lord Wrottesley, President, in the Chair. THE following communications were read :

"On the Geometrical Isomorphism of Crystals." By Henry James Brooke, Esq., F.R.S., Hon. M.C.P.S. &c.

The author commences by remarking that all the crystals at present known have been divided into the six following groups or systems: the cubic, pyramidal, rhombohedral, prismatic, oblique, anorthic.

He then states that he has constructed tables which accompany this paper of the minerals comprised in each of these systems, except the cubic, in a manner new, as he believes, to crystallography; and that the unexpected facts exhibited by the tables present that science under a new aspect.

The author explains briefly the language and notation he employs in discussing the results of the new tables.

It appears that the crystals in each system, except the cubic, are distinguished from each other by what are termed their elementary angles, that is by angles between particular faces of what may be termed elementary forms.

It is next observed that there is not in crystals any natural character which indicates an elementary or primary form, and it is shown that cleavage which Hauy regarded as such an indication, is only a physical character depending upon the degree of force with which the crystalline particles cohere at the surfaces of particular faces.

The question of high indices is also considered with reference to their influence on the choice of an elementary or primary form, and a general explanation is given of the nature of such indices.

The author then states that the most important of the facts presented by these tables, are the horizontal ranges of nearly equal angles, as shown in each system, and the general disagreement in the symbols hitherto assigned to the faces which make with some other face those nearly agreeing angles.