FAMILIES OF ROCKS 



how the forces of nature are at work to-day, can attempt 

 to describe the conditions under which rocks were laid 

 down before man ever trod them. 



In speaking or writing of the earliest stages of the 

 world's history we have to adopt what seems to be the 

 most likely history, modestly qualifying what we say by 

 adding that these speculations are only the fruits of an 

 inquiry that man has pushed beyond the ascertained facts. 

 But we are on firmer footing when we come to deal with 

 that portion of the globe which we can examine. The 

 crust of the earth has been found to consist of successive 

 layers of rock which, though far from constant in their 

 occurrence, and though often broken and crumpled by 

 subsequent disturbances, have been recognised over a large 

 portion of the globe. They are the earth's own chronicle 

 of its history. Had these rocks of the Geological Record 

 remained in their original positions we should have 

 known little of them, because only the most recent would 

 have been visible. Owing, however, to the way in which 

 the earth's crust has been twisted and cracked and broken, 

 portions of the bottom layers have been pushed up to the 

 surface, and the lower rocks have been inclined so that we 

 can examine their upturned edges. Instead, therefore, of 

 being restricted to examining a few hundred feet of earth 

 crust we can examine many thousand feet. The total 

 thickness of the rocks of Europe which contain fossil 

 remains has been estimated at 75,000 feet, or fourteen 

 miles. This vast depth of rock has been laid open to our 

 observation by disturbances, twists, contortions, upset- 

 tings of the crust. 



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