57 



Above features aid in determining geology of past times. 

 One may infer from the composition and structure of sea-laid 

 rocks the character of the waters in which they formed and 

 something of the nature of the lands which furnished the 

 sediments. A conglomerate or sandstone formation of ma- 

 rine origin tells of shallow, rather rough waters, and of 

 relatively high lands whose vigorous streams were able to 

 carry coarse material. Shallow water origin may be indicated 

 further by frequent alternation in the degree of coarseness, 

 by cross-bedding, ripple marks, or sun cracks. If the forma- 

 tion contains fossils, they are likely to be the remains of ani- 

 mals which inhabit water of slight depth. No one formation 

 would be apt to show all these features, but many formations 

 show several of them. A sea-laid shale formation implies 

 bottom waters too quiet to carry away mud. The presence 

 of ripple marks would, however, record some agitation of the 

 bottom water, while sun-cracked shales must have gathered 

 close inshore where wave and current action was weak and 

 streams did not furnish coarse sediment. If muds accumu- 

 lated extensively along the ancient shore, the adjacent land 

 must have been so low that its streams were sluggish and 

 therefore unable to carry coarse material. The fossils of 

 many limestones represent organisms which live only in clear, 

 quiet waters. Such limestones may have formed close to 

 shore if the land was sufficiently low, and protected from 

 wash by vegetation. 



In a similar way the composition and structure of non- 

 marine formations throw light on the conditions which existed 

 when the rocks were forming. 



The principles indicated here will be applied frequently in 

 the historical chapters, in determining the geography of 

 North America at the several stages of its development. 



QUESTIONS 



1. Acidic lavas are in general stiff er than basic lavas. Which 

 should you expect to be the leading type in (l)lava flows, (2) sills, 

 (3) laccoliths ? 



