GEOLOGY. 



their organisms. Hence they are characterised 

 by great absence of fossils. 



Gneiss, 1 so named from its thin layers, is a hard 

 crystalline rock, in extremely thin bands, often 

 twisted in a remarkable manner, and consists 

 mainly of the same materials as granite. Quartzite 

 is a metamorphosed sandstone, and is so called 

 from its quartzy appearance, arising from the fusion 

 of the sandstone. It must, however, be distin- 

 guished from quartz itself. The schists are named 

 from their chief ingredients. Clay-slate is that most 

 useful rock which, when split up into thin layers, 

 forms the familiar blue slates of our roofs, and 

 the slate and slate-pencil of our schools. These 

 rocks are often grouped together in nature, as in 

 the Highlands of Scotland, where they form the 

 mass of the mountains, and enter into the grand 

 and beautiful scenery of that picturesque region. 

 They occur most extensively in the older forma- 

 tions, as the Laurentian, Cambrian, Silurian ; but 

 it must be carefully noted that metamorphic rocks 

 are found throughout all the geologic systems up 

 to the Recent. For example, Carrara marble, 

 which was long thought to be primary, is meta- 

 morphic oolitic limestone ; and, in the Alps, ter- 

 tiary strata have been so changed as to be with 

 difficulty distinguished from the oldest rocks. 



It thus appears that the agencies now at work 

 on our own globe are adequately sufficient to 

 account for all the phenomena of the forming, 

 disturbing, disintegrating, elevating, depressing, 

 transporting, and transforming of the rock-masses 

 that form the crust of the earth. Such being 

 abundantly proved, and the laws of nature being 

 uniform and unchangeable, we are not only 

 warranted, but compelled, to infer that the same 

 influences were at work in these bygone ages, and 

 were the joint causes of the formation of our rock- 

 systems as they are now presented to our eyes and 

 subjected to our investigation. 



ROCKS AS RELATED TO TIME. 



The Length of Geological Periods, or Geological 

 Time. In studying geology, it is necessary to 

 have an accurate notion regarding the nature of 

 the periods spoken of. It is to be carefully noted 

 that, in geology, time cannot be measured by years. 

 When we examine any stratum of rock, with all 

 its enclosed organisms, it is natural to inquire how 

 long this mass of rock took to be deposited. We 

 can judge of this only in the following way. From 

 observation of river-action as at present exhibited, 

 we see with what extreme slowness rock-masses 

 are worn down into sand ; how a thousand years 

 make an almost imperceptible change on a boulder, 

 and even on the gravel by the shore. Yet we 

 know that the sandstone before us, often hundreds 

 of feet in thickness, is composed of grains of rock 

 ground down by water- action, transported by 

 rivers to the sea-bottom, and deposited there till 

 other strata were heaped upon it ; and that in 

 after-ages the grains united, and were hardened 

 by pressure into the rock we see. What incal- 

 culable ages, therefore, must this sandstone bed 

 have taken to be thus formed! The more we 

 think of these slow-working causes, the more are 



1 From Anglo-Saxon gnidan, to rub. 



we astonished at the enormous periods of time 

 that must have elapsed before the formation of even 

 the thinnest layer of rock. Geological periods 

 therefore, are quite indefinite in the matter of years. 

 This inability to assert a definite number of years 

 in regard to any formation, is no defect in the 

 science, for the knowledge of this would add 

 nothing to the conception we already have of the 

 immense periods presented to our contemplation 

 by geology. 



The Relative Ages of Rocks. When we speak 

 of the different ages of rocks, we can do so only by 

 comparison with others. Our ideas on this point 

 are merely relative. We can assert that one layer 

 must have been formed before another; or that, 

 after its formation, and before the deposition of a 

 certain other rock, a rise or fall in the strata took 

 place; or that, at a certain point in the series, a 

 volcanic eruption threw up a mass of igneous rock ; 

 and make like statements based on comparison of 

 the rocks with one another. Our conceptions, 

 therefore, regarding the connection in age between 

 the various rock-formations are merely relative, 

 one rock being proved to have been formed before, 

 or after, or during the formation of another. 



The Order of the Rock-formations. By long- 

 continued and widely extended observations in 

 various parts of the globe, based on numberless 

 data of composition, structure, inclination, and 

 fossil contents, geologists have been able to form 

 a definite list of the various rock-formations from 

 the earliest to the most recent, arranged in the 

 order of time. They have divided the whole of 

 the rocks composing the crust of the earth into 

 sections called 'systems,' and have subdivided 

 these again into ' groups,' in a certain well-defined 

 order. So that when a rock is presented to their 

 observation in any part of the globe, they can 

 state, with more or less certainty, the system to 

 which it belongs, and the period in the past history 

 of the earth at which it was deposited. Regarding 

 these rock-systems, one point is to be very strictly 

 noted. Suppose that we represent the various 

 rock-systems by the letters of the alphabet the 

 earliest by A, the second by B, and so onwards to 

 the last and most recent, represented by Z. Now, 

 the various rock-systems always stand in this rela- 

 tive historic order ; so that the formation indicated 

 by M comes after L, and before N, and cannot 

 occur in any other relation to these two systems, 

 wherever they may be found. At the same time, 

 certain formations, one or more, may be wanting 

 in some parts of the world, not having been 

 deposited there ; so that one or more systems may 

 not be represented in these districts. Thus, L and 

 M may be absent. What two systems will then 

 be found together? Certainly and unvaryingly, 

 K and N. But here the historic order is not vio- 

 lated, as it would be if N preceded K. The various 

 rock-systems are, therefore, always presented in 

 an unvarying succession in the order of their for- 

 mation ; although, in different parts of the globe, 

 certain strata, and even whole systems, may not 

 be found. 



CLASSIFICATION OF THE ROCKS 

 THE ROCK-SYSTEMS. 



We now proceed to describe, in the order of 

 their formation, the different kinds of rocks that 

 compose the crust of the earth, and their fossil 



