THE DEVONIAN PERIOD 361 



slow decay of these organisms is believed to have produced 

 most of the petroleum and gas which are now obtained from 

 the Devonian rocks in Ontario, Ohio, and Pennsylvania. 

 From the fact that the Hamilton formation thickens as it is 

 followed eastward it seems probable that the mud was largely 

 derived from lands along the present Atlantic slope. 



The basins coalesce. Toward the close of the Devonian 

 the epicontinental sea attained still greater extent. By the 

 spreading of the northwest Canadian sea southward and east- 

 ward, the western and eastern basins of the United States 

 seem to have been joined (Fig. 373). In the West and North- 

 west muds and oozes continued to be deposited, and from this 

 we may infer that in those remote times the western part of 

 America had none of its present rugged mountain ranges, but 

 that it was a flat or undulating lowland. As the upper 

 Devonian strata are traced eastward to Ohio and beyond, 

 they become increasingly thicker and more sandy. Wfc have 

 already learned that a change from ooze to mud and thence 

 to sand is to be expected as one approaches the shore line. 

 The Chemung formation, as these sandy shales are called, 

 grades finally into thick sandstones which contain few fossils 

 except leaves of plants and bones of fishes. It is from these 

 non-marine strata that the Catskill Mountains have been 

 carved. The imperfect assortment of the sediments suggests 

 that they were strewn by rivers rather than by waves, and 

 that the Catskill beds represent the alluvial apron built out 

 into the shallow sea on the west by streams which descended 

 from the highlands of the Appalachian continent. 



MIGRATIONS AND CHANGES OF THE SEA LIFE 



As the continuance of the clear sea over a comparatively 

 isolated province such as the Nevada region allowed the 

 animals which lived there to develop quietly along their own 

 lines of advance, so, on the other hand, the shifting relations 

 of land and sea and changing character of the sediments in 



