RECORDS OF MEETINGS 297 



may wonder why only one cycle of erosion seems to have been developed 

 in the limestone region since the late Miocene, while two are shown in 

 the crystalline area. The cutting of the main valley must have been 

 very rapid, while all secondary valleys became dry and many were 

 changed into closed depressions; so that the surface of the limestone 

 plateau suffered only slight changes by underground erosion. The great- 

 est changes certainly did occur in the caverns, and it would be possible 

 to trace the shifting of the base level by the study of some of them. The 

 fact that the main valleys carry water shows that they have reached the 

 level at which torrential circulation in the caverns is relayed by a com- 

 plete filling up of all hollows, some impermeable layers preventing a 

 deeper infiltration of water. 



It may be proposed to use the word Gausses in speaking of lime- 

 stone plateaus similar to these described here, when the surface is dry 

 and very few valleys carry water. One can distinguish between Low 

 Gausses, High Gausses and the Alpine Gausses, referring to the depth of : 

 the valleys, the surface of the plateau being more rugged on account of 

 stronger undermining by underground erosion, when the valleys are 

 deeper. We have described High Gausses. The Dordogne and Lot 

 cross the lower Gausses of Quercy. Alpine Gausses are frequent in the 

 limestone Alps of France (Vercors) and Austria (Steinernes Meer, 

 Todtes Gebirge, etc.). 



Dr. Jonas stated that the region discussed occupies part of the Boyer-- 

 town and Quakertown quadrangles, eastern Pennsylvania. The area lies 

 in the Appalachian mountains, locally called the Boyertown hills, and the 

 Piedmont plateau. The rocks of the Boyertown hills are a series of Pre-' 

 Cambrian gneisses, mainly of igneous origin. The sedimentary gneisses 

 are remnants of the Precambrian floor into Avhich are intruded an 

 igneous complex of granite, diorite, and gabbro. I^arrow diabase dikes 

 cut all the Precambrian rocks. The diabase is a fine grained, dark 

 rock with ophitic texture, composed of plagioclase, augite, pyrite and 

 biotite. Alteration of feldspar and augite has produced a dark green 

 schist. The rocks of the Piedmont plateau, lying southeast of the Boyer- 

 town hills, are sediments and diabasic intrusions of the Triassic age. 

 The sediments are the Brunswick conglomerate and shale, the upper 

 member of the Trias. Into them is intruded a diabase sheet, about 1,000 

 feet thick, locally called the Haycock Mountain sheet. It underlies a 

 prominent ^"^ridge" composed of rough hills which traverse the Quaker- 

 town quadrangle. The diabase is for the most part a light-colored, 

 coarse-grained rock, with ophitic texture, whose constituents are feld- 



