206 STRUCTURAL GEOLOGY. 



that, if they have not been greatly disturbed, their rela- 

 tive position indicates their relative ages, the uppermost 

 being of course the youngest. If, therefore, we have a 

 natural section of strata (an exposed sea-cliff or canon- 

 side), either horizontal or regularly inclined, it is easy to 

 make out the relative ages. But often the rocks are 

 folded and crumpled, and pushed over beyond the verti- 

 cal ; they are broken and slipped, and a large part worn 

 away by erosion ; they are covered with soil and hidden 

 from view ; so that to make an ideal section showing their 

 real relation is one of the hardest of geological problems. 

 Nevertheless, if this were all, we might still hope for per- 

 fect success. But all the strata are not represented in any 

 one place usually only a fraction. Thus, in New York, 

 and all the States westward as far as the Plains, only the 

 older portion of the record is found ; while in California 

 we have mostly the later portion. In many places the 

 record is still more fragmentary. The leaves of this book 

 are scattered about here, perhaps, nearly -a whole vol- 

 ume ; there, one or two chapters ; and yonder, only a few 

 leaves. The geologist must gather these and arrange 

 them according to their paging ; and then divide and 

 subdivide them into volumes, chapters, etc. Therefore, 

 although the order of superposition must, wherever it can 

 be applied, take precedence of every other method, yet it 

 must be supplemented by careful comparison of the rocks 

 in different localities with one another. There are two 

 means of comparison, viz., the character of the rock and 

 the character of the fossils. 



Comparison by Rock-Character. This method is of 

 little value except in contiguous localities. Sandstones 

 of similar character belong to nearly all times, and are 

 forming now. So, also, of clays and limestones. Coal 

 was once considered characteristic of a particular age, but 

 now is known to occur in strata of many ages. Chalk 

 was once supposed to be characteristic of the Cretaceous, 



