38 THE WONDERS OF GEOLOGY. Lect. I. 



are of various ages, and it is believed are sedimentary 

 deposits which have been altered by long exposure to a 

 very high temperature. 



2d. Transition Rocks. — These are super-imposed on the 

 primary, are more or less distinctly stratified, and contain 

 the fossilized remains of animals and plants. They re- 

 ceived the name of transition, because it was assumed that 

 they had been deposited at a period when the land and sea 

 were passing into a state fit for the reception of organized 

 beings. Modern researches have, however, shown that 

 like the primary rocks, these are strata which have been 

 altered by the effects of heat under great pressure ; hence 

 they are now termed met amorphic. 



3d. The Secondary Formations. — These strata have ori- 

 ginated, in great part, from the destruction of more ancient 

 rocks, and have been deposited in the basins of lakes, bays, 

 and estuaries, and in the profound depths of the ocean, 

 by the action of rivers and seas. They abound in the 

 mineralized remains of animals and plants ; the most ancient 

 enclosing zoophytes and shells ; the next in antiquity con- 

 taining, in addition, vegetable remains and fishes ; those 

 which succeed enveloping not only fishes, shells, zoophytes, 

 plants, and insects, but also bones of numerous species of 

 extinct marine and terrestrial reptiles, and of birds, and of 

 a few genera of mammalia. The Chalk is the uppermost, 

 or most recent of this class. As the secondary rocks have 

 manifestly been formed by the agency of water, it is clear 

 that they were originally deposited in horizontal, or nearly 

 horizontal layers or strata, although by far the greater 

 portion has since been broken up, and thrown in directions 

 more or less inclined to the horizon. 



For the convenience of study, this subdivision of the 

 rocks is still retained, as will hereafter be shown. To the 

 above groups modern geologists have added two others, 

 namely, the Tertiary, and the Alluvial Deposits. 



