« EelmineJätka »
God; they make us conversant with the multiplex organic forms through which life from its highest to its lowest phases performs its appointed functions in this world; and they invite us to surrey the master-piece of the Divine Architect in man, his visible image and likeness.
Of late years, among several others of note, Prof. Owen of London has distinguished himself by his published writings in the department of Homology. In 1848 he published his great contribution to this branch of science, entitled, “On the Archetype and Homologies of the vertebrate skeleton.” This was followed in 1839 by his work “On the Nature of Limbs.” Lately he has published a concise summary of his views, in a cheap form, in one of the volumes of " Orr's Circle of the Sciences,” which is described by a competent critic as a "little book both accurate and intelligible, and almost rendering any popular attempt in the same direction superfluous.” The subject has also been philosophically and skilfully handled in McCosh and Dickie's “ Typical Forms and Special Ends in Creation,” which, to a thinking reader, is really a valuable work. Dr. Ogilvie's book is much smaller and less ambitious than that of McCosh's, and aims at being more popular; and, we may add, more Zoological in its treatment of the theme. The author's great object is, as he states in his introduction, “not to advance new truths, but rather to gain additional currency for such as have a fair claim to be already established, and in particular to convey an idea of the laws of organization to those who, without making natural history a special object of study, may wish to have a right comprehension of its general scope. His style is very perspicuous and vigorous. Every page of the book gives evidence of independent thought and personal investigation. In nine chapters he treats of the various plans on which Animals are formed; of the Vertebrate type and its modifications in Fishes, Reptiles, Birds and Mammals; of the Articulate type, with its relations to the Vertebrate, and its special modifications; of the Molluscan and Radiate types; and of the mutual relations of the leading types of organization. Chapters eight and nine treat of the co-extensiveness of type and design with organic matter, and their bearing on Natural Theology. In the Appendix there is a valuable list of recent and accessible works on the various branches of Zoology. The work is illustrated with many admirable wood-cuts; and altogether it is a most acceptable addition to the student's library. of Natural History.
ARTICLE XI.—On the Microscopic Structure of some Canadian
Limestones. By J. W. Dawson, LL.D., F.G.S., &c.
[Read before the Natural History Society of Montreal.] Geology has shewn that over a great part of the earth's surface we can say, almost without hyperbole, “ The dust we tread upon was once alive.” Great and very extensively distributed beds of rock are of organic origin, made up of the remains of the hard parts of animals, and these often of very minute dimensions. In the bed of the sea, more especially in the coral regions of the Pacific, the Indian ocean and the equatorial Atlantic, such deposits are now manifestly in progress on a large scale; and in the archepelagos of the Pacific, the Bermudas, and the peninsula of Florida, we have examples of these modern formations elevated into land. Similar phenomena exist on a still greater scale in the Tertiary rocks; as for instance in the Nummulitic limestones, extending from the west of Europe almost continuously into India, built up into mountain masses in the Alps, Pyrenees, Carpathians and Himmalayas, and furnishing the materials of the Egyptian Pyramids, and of thousands of humbler structures. In the secondary period, the chalk and many of the oolitic limestones present similar phenomena. Similar organic rocks occur in all the members of the palæozoic series down to the lowest Silurian; and in these earliest periods of the earth's geological history, when organic CANAD. NAT.
' Vol. IV. No. 3.
life was perhaps young on our planet, the quantity of organic materials thus piled up into rock appears to have been as great as at any subsequent time. Of this some of the silurian limestones of Canada, and more especially the “ Trenton Limestone,” afford good illustrations, to which I desire in the present paper to direct attention; with the object, not of adding to the knowledge of their fossils, which have been so amply and ably illustrated by Prof. Hall and Mr. Billings, but of noticing the manner in which fragments of these fossils have been accumulated and cemented together into great beds of limestone.
The lowest of the silurian beds of Canada, the Potsdam sandstone, is wholly arenaceus, though with a few fossil remains. The Calciferous sandstone has a greater quantity of calcareous matter and more numerous fossil remains. The succeeding beds, the Chazy, Birds-eye, Black River and Trenton Limestone, are as a whole, of organic origin, and made up of more or less comminuted fragments of shells, corals, and crinoids, occasionally mixed or alternating with deposits of earthy matter. Above these limestones the Utica Slate consists mainly of muddy or earthy inatter, and in the Hudson River group there are frequent alternations of earthy matter with organic limestones. It thus appears that in Canada, the head quarters of lower silurian limestone of animal origin, is in the central members of the group, which, according to Sir W. E. Logan, have near Montreal a thickness of nearly four hundred yards, though much thinner in the western part of Canada, as for example in Lake Huron, where Mr. Murray estimates their thickness at only one hundred yards.
Perhaps the most continuous and fossiliferous of all these limestones is that named by the geologists of New York, from an excellent exposure at the waterfall of that name, the Trenton Limestone. It is largely developed in the vicinity of Montreal, and is thus described by the Provincial Geologist, as it occurs in the quarries near the Mile-end road. “In the vicinity of Montreal the lower part of the Trenton formation holds massive beds of gray granular limestone, from which a very large amount of the best building material used in the city has been obtained. The quarries opened on them extend obliquely across that portion of the Cote de la Visitation road, which is southward of the Papineau road, their general direction in respect of one another being about North and South. The beds vary in thickness from three inches up to three feet, and present an aggregate of from
od stary conditioting of thon's she
eight to twelve feet. In successive quarries, from the one to the other of which the beds can be traced with considerable certainty, individual beds appear occasionally to change in thickness, a massive one gradually dividing in the strike into two or more, or several thin layers uniting into a solid mass. Slight changes in the color also occur, giving shades of lighter and darker gray."
This gray granular crystalline stone, the texture of which we may see by picking up a chip at any mason's shed in Montreal, is wholly an organic rock, consisting of the hard parts of marine animals, in a fragmentary condition. In some specimens, joints of those curious stalked star fishes, the crinoids or stone lilies, predominate. In others a little branching coral, the Monticulipora dendrosa of Mr. Billings, but ranked as a variety of the Chaetetes lycoperdon by Hall, is more plentiful. In others, creatures of higher organization than the true corals, the Polyzoa, have contributed countless fragments of a delicate structure, which may often be seen spreading over the limestone in flat branches, marked with little holes or cells like perforations of pins, and belonging to the genus Stictopora of Hall,* probably the Ptilodictya of the European Paleontologists. The limestone does not merely contain these organisms; it is made up of them, sometimes entire or in large pieces, but more frequently in minute fragments from one tenth to one hundredth of an inch in size. Its present solid condition is due to clear transparent calc-spar or carbonate of lime, deposited by water in the interstices and cavities of the fragments, like the “congealed water" of Bermuda or the stalagmite of limestone caverns. This substance being perfectly crystalline, has given its own character to the mass, which thus breaks like marble with multitudes of shining surfaces. Under the microscope, however, the true character of the material becomes at once apparent, and the animal fragments, rendered distinct by the remains of their organic matter in a carbonised condition, are seen immersed in the transparent calc-spar, like pieces of potted meat in animal jelly.
To prepare the specimens for the microscope, it is necessary only to select thin fragments, polish them smooth on one side, then attach the smooth surface by any transparent cement to glass, and grind down the opposite side until the limestone is reduced to a
• Especially S. Acuta.
thin transparent film. A low power is sufficient to show the general forms and nature of the fragments, but they are often so beautifully preserved as to display their most minute structures when examined with bigh powers.
With the view of ascertaining whether there is any difference of material in different parts of the beds, I selected from one of the quarries, containing two thick beds with some intervening and overlying thin shaly layers, specimens representing the overlying shaly limestone, the material between the two beds, and the upper, middle, and lower portions of each bed. Duplicate preparations of all these specimens were kindly made for me by Mr. E Murphy, of this city.; and on being examined they afforded the following results : 1. Above upper bed. Very small fragments of crinoids and shells witb
numerous minute and probably young univalve and bivalve shells, in patches in a paste containing black earthy and organic
matter. 2. Upper part of bed A.-Principally joints of crinoids ; some fragments
of corals, especially Ptilodictya and Monti
culipora, and shells. 3. Middle of Bed A. Similar to the last but more corals. 4. Bottom of bed A.-A still greater proportion of Ptilodictya and
Monticulipora. 5. Between A. & B.--Crinoids and Monticulipora in about equal pro
portions, some Ptilodictya. 6. Upper part of bed B.-Similar to last, but still more corals. 7. Middle of B.-Principally Monticulipora and Ptilodietya, but still
many crinoids. 8. Bottom of B.-Almost wholly Monticulipora and Ptilodictya.
It thus appears that the only material difference in these specimens is the great prevalence of crinoids toward the top, and of corals and Polyzoa toward the bottom. It is proper to add that, though the families and genera pamed above largely predominate, there may be detected in any specimen fragments of shells of brachiopods and gasteropods, and of corals of other genera than those named, though in comparatively small quantity.* With the exception of the first specimen, none exhibited more than traces of eartby or aernaceous matter.
The fine grained earthy limestone, marked No. 1, and which,
• According to Mr. Billings, Cystideang have also been important contributors; but in my examinations their remains are not distinguished from those of the Crinoids.