WESTERN COUNTII?S OF PENNSYLVANIA 53 



limestone division except near the top. That, however, is the same with 

 that which in the other sections was taken as the bottom of the shales. 

 The silicious limestone is massive except 10 feet at the bottom.* These 

 measurements were made at about 40 miles southwest from the Somerset 

 County exposures under the Viaduct axis. 



Western counties of Pennsylvania. — Returning now to the north, one 

 finds in Crawford county of Pennsylvania the Shenango shales of Doctor 

 White, which there vary from 35 to 60 feet and consist of gray to brown 

 and blue shales. The thickness becomes 45 feet near Kinzua in Warren 

 county, east from Crawford, while southward it is 60 feet near Tidioute 

 in Venango and 47 feet near Sharon in Mercer.f Mr Carll gives 50 feet 

 for Warren, northwest from the Allegheny river, but from 120 to 150 feet 

 in the southeastern part of the county. J Fossils occur occasionally 

 within Crawford and Erie counties, which, according to White, belong to 

 types characterizing the Chester group. No limestone appears in these 

 shales and they evidently represent the whole sedimentation for the 

 Mississippian in this area. 



The relations in the southern counties of western Pennsylvania are 

 shown in the tabulated series of well records given by Mr Carll in his 

 report for 1886. In Crawford county, the Shenango shales are 50 feet; 

 in Venango, 60 feet; in Clarion, east from Venango, 172 feet; in Butler, 

 90 to 194 feet, increasing southwardly; thus far no limestone. The 

 series is thickening and evidentl}^ we have here the upper and the lower 

 shales, between which the limestone should make its appearance. At 

 Pittsburg, about 30 miles west from the Conemaugh gap, through Chest- 

 nut hill, the succession differs in some respects from that seen in the gap ; 

 the lower shales are here, 44 feet, underlying the silicious limestone, 56 

 feet, on which rests the upper limestone, which is shaly and 23 feet 

 thick, but the upper shales are absent, so that the Pottsville sandstone 

 is let down directly on the limestone. A similar section is shown at 

 Murraysville, about midway between Pittsburg and the gap, where, how- 

 ever, the upper shale is present, the thicknesses being, shale, 30 feet ; 

 limestone, 30 and 90 feet ; shale, 60 feet. In Mount Pleasant township of 

 Washington county, about 20 miles southwest from Pittsburg, the lower 

 shale is still present, 27 feet, the upper shale, only 10 feet, while the 

 limestones have diminished to 13 and 29 feet. This, clearly, is not far 

 from the western limit of the limestones, for at the S. B. Phillips well 

 number 1, near McDonald station, 10 miles northwest, no limestone is 

 present and the whole interval from Pottsville to Pocono is but 39 feet. 



*I. C. White : Catalogue of West Virginia University, 1882-3, p. 50. 

 tl. C. White : Geology of Erie and Crawford counties (Q 4), 1881, p. 78. 

 t J. F. Carll : Geological report on Warren county (I 4), 1883, p. 193. 



