16 GEOLOGY, 



formation is found in the remains of life entrapped in the water-laid 

 sediments. Whatever life existed in the waters in which the sediments 

 were deposited was liable to burial, and if it was possessed of hard 

 parts, such as bones, teeth, shells, hard integuments, etc., these parts, 

 or at least their impressions, were likely to be preserved in the sedi- 

 ments. Even tracks and imprints of perishable parts are sometimes 

 preserved. All these relics, which we call fossils, give indications of 

 the kinds of life which existed when the beds were formed. The fossils 

 of the youngest beds show that the life which existed when they were 

 deposited was quite like that of the present time. The fossils of the 

 next older and lower beds show greater departure from present types. 

 This series of changes continues downward as lower and lower beds are 

 studied, until beds at considerable depths contain no relics of existing 

 species but, in lieu thereof, forms of more primitive types. Some of 

 these earlier types are clearly the ancestors of more modern forms, 

 while others seem to have no living descendants. Going still deeper, 

 the fossils indicate life of more and more primitive types, until they 

 depart veiy widely from the living forms, and seem to be but remotely 

 ancestral. So the beds may be followed downward until the lowest, 

 which contain distinct evidences of life, are reached. 



It should be understood that it is not possible to proceed directly 

 downward through the whole succession of bedded rocks, but that 

 the edges of the various beds may be found here and there where they 

 haVe been brought to the surface by warpings or tiltings, or exposed 

 by the wearing away of the beds which once overlay them. The full 

 ^e^ies of strata is made out only by putting together the data gathered 

 thi^Oughout all lands, and even when this is done an absolutely com- 

 plete series cannot yet be made out or, at least, has not been. 



The crystalline rocks. — The crystalline rocks which would appear 

 if the mantle rock were removed are of two tj^pes, igneous and meta- 

 morphic. Igneous rocks may be loosely defined as hardened lavas. 

 Met amorphic rocks are those which are greatly changed from their 

 original condition. Either stratified or igneous rocks may become 

 metamorphic. 



Igneous rocks sustain various relations to the stratified rocks, 

 as illustrated by Fig. 2. From these relations it is possible to tell 

 something of the order of their formation. Where the stratified rocks 

 are broken through by lavas, it is obvious that the stratified rocks 

 were formed first, and the lavas intruded later. Lava sheets intruded 



