IF". A. Verwiebe — Berea Formation. 51 



thoroughly studied and the productive formations described in 

 many Federal and State publications. Of course it must be 

 granted that many difficulties are encountered by the geolo- 

 gist in attempting to unravel the stratigraphy of rocks totally 

 concealed. However, the many accurate and detailed well 

 records now available enable a person, thoroughly familiar 

 with the formations on the outcrop, to make rather satisfactory 

 correlations. 



Of the formations involved in this discussion the following 

 in descending order are locally important as oil or gas horizons : 

 Shenango sandstone, Berea (Corry), and the Venango 1st and 

 2d sands. Of these the first is generally identified with the 

 " Big Injun " of well drillers. No doubt it frequently includes, 

 besides the Shenango sandstone, also the Shenango shale as 

 well as some of the overlying Pottsville (where the Mauch 

 Chunk is absent) and some of the upper Meadville (as in sec- 

 tion M). Beneath this, and with an interval varying from 150 

 feet in the northern part of the field to about 350 feet in the 

 southwestern part, occurs the Berea sandstone. It carries oil 

 and gas in some parts of the Pennsylvania field, but is leather 

 unimportant in other parts. For this reason it does not always 

 have a prominent place in the well records. However, the 1st 

 Venango oil sand, which is widely known as the Hundred-foot 

 sand in Armstrong, Butler and Beaver counties, or as the Gantz 

 and Fifty-foot sands in "Washington and Green counties, is a 

 very important source of oil as well as gas ; and this furnishes 

 a very reliable datum plane when accurately determined. By 

 using the latter in addition to the base of the " Big Injun," it 

 is possible to trace the Berea sandstone with considerable defi- 

 niteness, southwest, from its outcrop on the Allegheny into 

 southern Pennsylvania, and also into Ohio and West Virginia. 



Explanation of fig. 1. — The sections in fig. 1 (opposite page) were chosen 

 to illustrate the character and thickness of the Berea formation along an 

 east and west line, from its type locally east to the Allegheny River. Their 

 relative location is indicated on the accompanying map (fig. 2). Sections A, 

 B, C, and D are taken from Prosser's report on the Devonian and Mississip- 

 pian (Geol. Survey of Ohio, Bull. 15) and may be found in detail on pages 

 485, 125, 32i, 433 respectively. Sections G and H were taken from White's 

 report on Erie and Crawford counties (Pa. 2d Geol. Survey, vol. Q4, p. 164 

 and 182). Sections E, F, I, L and M were made by the author and the last 

 three will be found in greater detail at the end of the paper. Section K is 

 taken from Butts' work as explained in the above pages. 



The top of the Berea (Corry) was used as the base line in platting the sec- 

 tions. The symbols used are the conventional ones. The legend in the 

 upper left hand corner gives the equivalents of the Roman numerals used. 

 For the correlation of these, see page 43. (The scale is 175 feet = 1 inch.) 



A rather interesting feature brought out by this figure is the irregular thick- 

 ness of the Berea, which may be construed as evidence of a disconformity at 

 the base. Another fact well shown is the lenticular character of the Cusse- 

 wago sandstone. 



