784 



GEOLOGY. 



CHAPTER Xin. 



GENERAL INDICATIONS. 



HAT our globe existed through cycles of long duration be 

 fore man trod upon its surface, so that the time of its intel 

 lectual occupation is but a handbreadth in comparison with 

 its whole length of days, is a point of which the evidence 

 derived from the study of the terrestrial crust is irresistible. 

 It is difficult to convey an adequate conception of the 

 HJl strength of that evidence to those who are not practical 

 observers, but some parts of it are very striking and in 

 telligible. Let us imagine a person conversant with the 

 history and details of architecture to go into an old manor- 

 house, and he will be able approximately to estimate its 

 antiquity, by observing its style to be Elizabethan, or to 

 belong to the early Tudor period. But if, upon a more 

 minute examination, he should discover a part diverse from 

 the rest, a crypt, a doorway, or a keep, of a Norman or 

 Saxon character, then his thoughts will go back to a more 

 remote era than the date when the edifice in its present 

 state was formed. A process of recurrence from one 

 period to another more ancient is forced upon us by an 

 actual inspection of existing rocks. Let us consider, for 

 example, the old red sandstone, plainly a deposition in 

 water of fine sand, clay, and gravel, tinged with oxide of 

 iron. We know by what a slow process a mass of matter 

 is formed by aqueous deposition, the aggregation and hard 

 ening of a very thin stratum requiring a longer interval 

 than our threescore years and ten. Macculloch states, that 

 a Scottish lake does not shoal, or deposit mud and marl to 

 remain at the bottom, at the rate of half a foot in a cen 

 tury, yet Scotland presents a vertical depth of far more than 3000 feet of this 

 single formation, the aggregation of which, according to the ratio, involves a cycle equal 

 to a hundred times the former presumed age of the earth. But in the lower range of the 

 sandstone we meet with conglomerate, bearing similar relation to the mass in which it is 

 imbedded, which a Saxon arch incorporated in an Elizabethan building would have to 

 the edifice. The conglomerate consists of water-worn pebbles of granite, quartz, and 

 other material, having no analogous character to that of the overlying formation. Here, 

 then, we are constrained to go on to a still more remote antiquity, for the pebbles must 

 have existed in their parent rocks, been detached from them, and eroded by long rolling 

 on a solid bottom under water, before the deposition of the superjacent sandstone com 

 menced. Now, let us remember, that the tertiary, cretaceous, oolitic, saliferous, and 

 carboniferous series of strata, have been successively produced subsequent to the old red 

 sandstone, while the silurian rocks, the clay slates, mica-schists, and gneisses, were formed 

 anterior to it by similar aqueous deposition, and we are compelled to extend the chro 

 nology of the earth's adamantine pavement to a period in comparison with which the 



