398 HISTORICAL GEOLOGY 



banks of mud and ooze accumulated. In consequence of 

 compression at a later time, the sediments are now folded 

 slates and schists. 



Erosion of the new Appalachian Mountains. No surely 

 marine beds of Triassic age are exposed in the eastern part of 

 the continent. The land probably extended out even beyond 

 the present Atlantic shore. 



The growth of the young Appalachian Mountains seems 

 not to have ceased entirely in the Triassic period. As the 

 Paleozoic rocks were folded, the eroded surface of old Appa- 

 lachia was likewise warped, and, in the broad downwarps, 

 streams continually spread the abundant sand and silt which 

 they were removing from the mountains. The deposits on 

 the low slopes and bottoms of these basins eventually reached 

 a thickness of thousands of feet ; and, as is common among 

 rapidly accumulating sediments, the particles were not well 

 assorted. Alternations of sandstone and shale of varying 

 colors are characteristic of the Newark formation, as these 

 beds are called. Red is the predominating color. 



Locally, as in southern Virginia, the Triassic rocks include 

 a few beds of coal, which bespeak the growth of marshes in 

 the low grounds. The fact that these marshes existed prob- 

 ably means that the climate of the Atlantic seaboard was less 

 arid than that of the West. 



Volcanic eruptions in the Atlantic slope. About the time 

 the Newark sediments were being laid down, eruptions of 

 basaltic lava occurred in the same district. Some of the 

 basalt sheets rise obliquely through the strata, thus proving 

 that they were squeezed into the rocks as intrusions. Others 

 have cindery surfaces and are overlain by sandstones which 

 are not altered at the contact with the lava. Here, evidently, 

 the flows were poured out upon the surface and afterwards 

 buried beneath sediments. 



Being harder than the sedimentary strata, the lava sheets 

 have been left as ridges in the subsequent wasting of the 

 surface. The palisades of the Hudson and such heights as 



