THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



635 



and unstratified rocks, indicates their connection with an igneous cause ; and, in com- 

 mon with other veins, the two principles of simple injection and chemical segregation 

 have doubtless operated in their production. 



The preceding division of rocks into two classes the stratified and the unstra- 

 tified was first made by Leibnitz, in the year 1680. Optically, the stratified im- 

 mensely preponderate over the unstratified, the latter being supposed to occupy not 

 a tenth part of the earth's surface. The proportion of each varies remarkably in 

 different countries; the unstratified masses in Great Britain, according to Macculloch, 

 not covering a thousandth part of the superficies of the island, while in Massachusetts 

 they occupy nearly a quarter of the surface. But it must be remembered, that what 

 meets the eye, in this case, is no criterion of the relative magnitude of these two parts of 

 the crust of the globe ; for the whole of the interior shell, to a great depth, is probably 

 constituted of masses of unstratified rock, kindred to those which in former ages, in an 

 ebullient state, have disrupted the strata, and are now open to our inspection. 



The subject we are now considering the structure of the exterior mass of the earth 

 necessarily requires a division of rocks into two more grand classes the fossiliferous and 

 the non-fossiliferous, or those formations which contain animal and vegetable remains, 

 and those in which no organic remains have been discovered. The non-fossiliferous 

 include all the unstratified rocks, with the earliest two or three groups of strata the gneiss 

 and mica-schist systems. But it by no means follows, that because the latter sedimentary 

 strata exhibit no organic remains, therefore organised nature, animal or vegetable, was 

 not coincident with their deposition ; for all vestiges would be destroyed by the igneous 

 action with which they have been in immediate contact, and which has given them a 

 metamorphic character. The fossiliferous class embraces all the stratified formations 

 excepting those just mentioned, the remains of animal life vastly preponderating over the 

 remains of vegetables in the older strata, except in the beds connected with the coal a 

 fact which may be sufficiently explained by the easy destructibility of vegetable fibres. 

 In the Alps, the fossiliferous rocks are found at the height of from 6000 to 8000 feet 

 above the level of the sea ; in the Pyrenees, nearly as high ; and in the Andes, at the 

 height of from 16,000 to 18,000 feet. In examining a formation of very limited extent, 

 the same specific organic remains may be found universally diffused through it ; but if the 

 formation has a larger area, extending into different countries or hemispheres, the specific 

 resemblance between the organic contents will be found to diminish according to the 

 distance ; and excepting in a few particular instances, no wide diffusion of species will be 

 found in contemporaneous rocks. This is a highly interesting fact, since it manifests the 

 coincidence between the laws of the animal and vegetable kingdoms, in immeasurably 

 distant ages and at the present epoch ; groups of species being confined to distinct botanical 

 and zoological provinces. In the more recent formations, examples of the remains of 

 species identical with those which mark the existing condition of nature occur, but the 

 proportionate number of these and extinct species becomes less as we descend through 

 the six or seven tertiary beds, till we arrive at the chalk, in and below which no species 

 are observed which can be identified with any now living, though, according to Koferstein, 

 a German writer in 1834, the species of organic remains described in rocks below the 

 tertiary strata amount to upwards of 9000. 



It is an extraordinary fact, that whole masses of the solid materials of the globe appear 

 to be composed almost entirely of the remains of animals or plants. Ehrenberg, the 

 Prussian naturalist, mentions a bed in Germany, fourteen feet thick, made up of the 

 shields of animalculae, so small, that it would require 41,000,000,000 of them to form a 

 cubic inch. But mountains, hundreds and even thousands of feet high, are essentially 

 constituted of organic matter ; and some idea of the enormous amount entering into the 



