58 Geology of the Mohongahela Valley. 



the mountains and the roads which traverse these ranges. A large 

 proportion of the pebbles found lining the shores of the Ohio river, 

 for hundreds of miles below, have been in ages past, brought by the 

 currents from these mountains. No other rock, within my knowl- 

 edge is constructed of materials that could possibly furnish these 

 pebbles — many of them being of white and black quartz, green- 

 stone, graywacke and sienite ; the relics of former and older rocks, 

 which were broken down and rolled by the waves of the ancient 

 ocean, and in progress of time mingled with the sands on its shores 

 and agglutinated into conglomerate. The Youghiogany river takes 

 its rise in a mountainous region in the N. W. corner of Maryland, 

 and passing through a broken and hilly country, unites its waters 

 with the Monongahela, a few miles above Pittsburgh. The face of 

 the country, does not differ materially from that on the latter river. 

 In its course, it passes both the Chesnut and Laurel ridge of moun- 

 tains. Some of the richest beds of bituminous coal are found on its 

 head branches. 



Geology of the Valley of the Monongahela. 



The rock formations through the whole extent of this valley, are 

 recent secondary, consisting generally of sandstone. In the compo- 

 sition of most of them, the argillaceous portion is very abundant, giv- 

 ing an argillaceous character to the soils which form the surface of the 

 earth, and aifording nourishment and support to the vast forests 

 which every where clothe its hills. The prevailing color of the sand- 

 stone rocks is light grey, although often tinged with brown or yel- 

 low by the oxide of iron, which material is more or less abundant 

 in nearly all the strata. The dark brown or red variety, known as 

 the "old red sandstone," is seldom seen, except in some of the 

 mountain ranges, and is strictly a transition rock. The structure is 

 more close and fine grained, near the base of the hills, and coarser 

 as it approaches their summits. In many places and in particular 

 strata, silver colored mica, in small plates makes a conspicuous fig- 

 ure in its composition. In other strata, although rarely, the mica 

 is yellow. It was originally deposited in horizontal beds, which are 

 changed, more or less, from this position, by their proximity to ran- 

 ges of high hills or mountains. The inclination of the strata, being 

 evidently influenced by the rise of the country, as it approaches 

 the Alleghany ranges, indicates that some force from below acting 



