igo THE BOOK OF NATURE STUDY 



and the Chalk, shall have their correct historical position. All 

 the " historical documents " which may be contained in the 

 strata can be dated by means of this time-scale. Then when the 

 " documents " are deciphered and interpreted they will furnish 

 some contribution to the history of the region in which they 

 occur, and thus give a part of the history of the world during 

 the time occupied in the formation of the stratified rocks. 



By piecing together a record in this way from various parts 

 of the world, it is found that if all known strata were at any place 

 to be seen lying horizontally one over the other, they would be 

 350,000 ft. thick, and a boring to pierce them would have to be 66 

 miles deep . Records of the oldest history would be brought up from 

 the bottom of the borings, and of later history from nearer to the 

 surface. It is very occasionally necessary to resort to boring to 

 acquire evidence of this kind. But the geologist takes advantage 

 of the tilting of rocks to study the older ones, where by folds and 

 denudation they have been brought near the surface. He reads 

 the succession by means of the dip, and so works out the sequence 

 of events from the records preserved in the rocks. Thus the 

 popular impression that things buried very deeply in the earth are 

 necessarily very old, and those near the surface very new, is 

 wrong, but it is so prevalent that much care must be taken to 

 eradicate it. 



The records in the rocks fall into two main groups, biological 

 and physical. The biological records are obtained from the fossil 

 animals or plants which may chance to have been preserved. 

 The physical records depend on the nature and composition of 

 the rock, its position and structure, its relations to others, its 

 range, and to a very considerable extent on the interpretation of 

 its fossil contents. A vast amount of detail on both these heads has 

 been patiently collected and laboriously recorded and described, 

 and some important general conclusions have been drawn from 

 it. 



Physically, it has been shown that practically all rocks have 

 been formed by agencies such as may now be seen somewhere or 

 other in operation ; that vast physical changes in land, water, and 

 climate have occurred in the past, but they have been exceed- 

 ingly slow and gradual in operation ; that the story of destruction 



