6 [January, 



Nearly the entire cities of Pittsburgh, Allegheny and Cincinnati are built 

 upon this remarkable deposite. 



From Pittsburgh to Wilkinsburg, seven miles east, it is seen stretched along 

 the northern shore of the Monongahela river, and nearly two miles in width. 

 To this remarkable locality I will now briefly direct attention. 



Passing from Greensburgh to Pittsburgh, along the turnpike, it will be observed 

 that the last anticlinal roll of the Allegheny chain of mountains is at Grapeville, 

 four miles west, and the last synclinal trough, which is but imperfectly formed, 

 is about three miles further west. From this point the beautiful undulations 

 which are so characteristic of the Allegheny chain, die away, and the rocks 

 assume nearly a horizontal position. Their horizontality, however, is only appa- 

 rent, for they really rise, at an almost inappreciable angle, westward. From 

 this point to within seven miles of Pittsburgh the general geological and topo- 

 graphical structure of the country remains nearly the same. 



At the summit of the hill, which overlooks the town of Wilkinsburg, standing 

 upon the limestone which is super-imposed upon the great Pittsburgh seam 

 of coal, the observer beholds, more than a hundred feet beneath him, a beautiful 

 and extensive alluvial plain, bounded on the north by hills, which separate it 

 from the Allegheny river valley, and which contain the coal and limestone to 

 which allusion has just been made, and on the South by the Monongahela river. 



The whole of this great seam of coal and the incumbent limestone, to the 

 depth of three hundred feet, have been swept away by a process of denudation, 

 and their places partially supplied by an alluvial deposite over one hundred feet 

 in thickness, and about the same height over the present bed of the Monongahela 

 river. 



As the Pennsylvania Railroad passes over this deposite I shall avail myself of 

 the measurements which are given in one of the last annual reports of the Com- 

 pany. 



The elevation of the Ohio river at Pittsburgh, above tide, is given at 700 

 feet. The elevation above tide at Wilkinsburg is 922. There would, there- 

 fore, be a difference of tidal elevation between Pittsburgh and Wilkinsburg of 

 222 feet. After substracting 22 feet feet for the fall of the river between these 

 two points, we would have 200 feet for the depth of the deposite. 



If we assume that the shale and limestone upon which this vast deposite re- 

 poses, and which will be presently described, are from 200 to 300 feet beneath 

 the Pittsburgh seam of coal, we will be enabled to form some idea of the enor 

 mous denuding process, by which this river excavated its channel, and after- 

 wards deposited upon its ancient bed, in some places, over two hundred feet of 

 sedimentary matter. This is, of course, only an approximative estimate ; but 

 from all the data which I have been enabled to procure, the average depth may 

 be safely assumed at from one hundred to two hundred feet. 



In consequence of the great depth of this deposite at Wilkinsburg, I, at one 

 time, conjectured that the Allegheny and Monongahela rivers met, at some 

 antecedent period, and formed the Ohio, either at this point or at East Liberty, 

 which are from five to seven miles East of their present junction. To satisfy 

 myself on this curious point, I crossed the plain at Wilkinsburg, three quarters 

 of a mile, to the base of those lofty hills which separate the two great alluvial 

 valleys, and found that the whole region was composed of river pebbles, sand 

 and loam, with which were mingled fluviatile and terrestrial shells. 



Near the Frankstown road, which is nearly two miles in a straight line from 

 the Monongahela river, I saw some men excavating a well. They had gone 

 down to the depth of twenty or thirty feet, and had passed through nothing but 

 pure river sand and pebbles, meeting occasionally only with an Anodonta or a 

 Unto. 



Near this point I crossed the dividing hills, and descended along a narrow 

 path to the road which winds along the bank of the Allegheny river. Here I 

 found this ancient deposite presenting: nearly the same appearances, and of about 

 equal magnitude to the one on the Monongahela side. In the neighborhood of 

 Laurenceville it is of great breadth, and of not less than two hundred feet in 

 hickness. 



