1 66 ELEMENTS OF SCIENCE 



depth, therefore, is purely a matter of inference from 

 the arrangement, superposition, and inclination of the 

 different strata, as seen at or near the surface. 



After penetrating a moderate distance, the tempera- 

 ture of the earth's interior has been found to augment, 

 the greater the depth explored, at the rate of i Fahren- 

 heit, sometimes for every 45 feet and sometimes for every 

 70 feet approximately. 



The various strata of which the earth's crust is com- 

 posed were, of course, deposited at successive times, and 

 the time of the deposition of each is called its " period " 

 or " epoch." But for subsequent disturbance, the most 

 ancient strata would always be deepest, and superposition 

 would, in all cases, plainly indicate relative novelty. As 

 it is, we have often to examine carefully in order to 

 discover the real order of deposition, but this once 

 discovered, depth is equivalent to age, and vice versa. 

 The uppermost and most recent accumulations of sands, 

 clays and gravels, form what are called the "recent 

 deposits," and these are not counted as forming any part 

 of the proper geological strata, and are not represented 

 in ordinary geological maps, but are there disregarded. 

 The strata beneath these recent deposits are classed in 

 three great groups, belonging respectively to three great 

 epochs. The deepest and most ancient group comprises 

 the strata called Primary or Palceozoic. The second or 

 middle group of strata is called Secondary or Mesozoic. 

 The uppermost and least ancient group consists of strata 

 called Tertiary or Cainozoic. The "recent deposits" 

 really belong to this last-mentioned group, and we may 

 be said to be still living in the Tertiary period, which has 

 succeeded the only two earlier periods of which as yet we 

 have evidence namely, the secondary and the primary 

 periods or epochs. 



