Septembeb 24, I'JOn] 



SCIENCE 



415 



sentation of papers were held on the first day, 

 and a field trip to typical localities of the pre- 

 Cambrian and early Paleozoic rocks of the region 

 on the second day. After an address of welcome 

 by Dr. Edward J. Nolan, secretary of the academy, 

 seven papers were read, as follows: 



The Lower Cambrian of Laiicaster County, Pa.: 

 H. Justin Roddy, State Normal School, Mil- 

 lersville, Pa. 



The rocks of the Lancaster Valley comprise 

 quartzite, argillite and limestone, in all of which 

 abundant fossils have recently been discovered, 

 including many of the typical forms of the 

 Olenellus fauna. The argillites, in particular, 

 contain magnificently preserved examples of Ole- 

 nellus thompsoni and Holmia vxdcotti. No Mid- 

 dle or Upper Cambrian fossils have as yet been 

 found, but the limestones are overlain on the 

 north by shales of " Hudson River " type, at the 

 base of which traces of Ordovician forms have 

 been observed. Because of complicated structure, 

 the thickness of the Lower Cambrian is not cer- 

 tainly known, but it probably exceeds 3,000 feet. 



The Pre-Oambrian Gneisses of the Pennsylvania 



Piedmont Plateau: Miss Bascmm, Bryn Mawr 



College. 



Of the crystalline rocks of this district the 

 gneisses present the more serious difficulties in 

 the determination of age, origin and stratigraphic 

 relations. 



There has been determined the following suc- 

 cession of pre-Cambrian gneisses: hornblende 

 gneiss, granite gneiss, Wissahickon mica gneiss, 

 Baltimore gneiss. 



The Baltimore gneiss, underlying the Paleozoic 

 series, to the lowest member of which it has fur- 

 nished debris, exhibits two facies : a massive facies 

 presumably of igneous origin and a sedimentary 

 facies peripheral in position. 



The Wissahickon mica gneiss exhibits many 

 facies, due to the injection and impregnation of 

 a sedimentary formation of somewhat varying 

 composition, but always characterized by an ex- 

 cess of mica. This gneiss is adjacent to, or over- 

 lying the Paleozoic series, but is considered to be 

 separated from them by a thrust fault for the 

 following reasons : 



1. While the gneiss persists over great areas, 

 the adjacent Paleozoic series change from one 

 member of the series to another and in the thick- 

 ness of single members. 



2. The gneiss shows a coarser crystallization 

 than the adjacent Paleozoics. It is contrasted 



with the Ordovician mica schist, a formation of 

 similar composition with which it is in contact 

 for long distances, from which the gneiss can 

 always be separated by a greater degree of meta- 

 morphism and by structure. 



3. The Wissahickon gneiss, like the Baltimore 

 gneiss, is thoroughly intruded by chonoliths of 

 granite, gabbro, pyroxenite and peridotite, which 

 are not found intruded in the Paleozoics. 



The hornblende and granite gneisses are mani- 

 festly igneous in origin, intrusive in character and 

 younger than the other gneisses. 



The Medina and Shawangunk Problems in Penn- 

 sylvania: A. W. Gbabau, Columbia University. 

 The Formation No. IV. of the Pennsylvania 

 Surveys is not all of the same age, as formerly 

 supposed, but comprises two entirely distinct 

 groups of formations. The lower of these in- 

 cludes the Bald Eagle conglomerate, well exposed 

 in the westernmost of the Appalachian ridges, 

 which is of Upper Ordovicic, approximately of 

 Eden age; the Juniata red-beds, corresponding to 

 the Queenstown shales of western New York, of 

 late Lorraine and Richmond age; and the Tus- 

 carora sandstone, the equivalent of the true 

 Medina, marking the base of the Siluric. In the 

 easternmost of the ridges, the Blue Mountain, the 

 conglomerate is the Shawangunk, which is known 

 to be of Salina age; and this is followed by the 

 Long\vood shales and they in turn by the Lewis- 

 town limestone, which is uppermost Siluric. The 

 conglomerates and shales are believed to be of 

 continental origin, representing the alternation of 

 torrential deposits with flood plain and seolian 

 deposits under semi-arid climates. Their geo- 

 graphical distribution shows them to have the 

 form of great alluvial fans, deposited by rivers 

 flowing from the southeast; and the occasional 

 intercalated fossiliferous beds represent the tem- 

 porary advance of the sea upon the margins of 

 the fans. 



The Buried Gorge of the Hudson River: W. O. 

 Ceosbt, Massachusetts Institute of Technology. 

 Glacial Erosion in Great Britain, France and 

 Switzerland: Douglas Wilson Johnson, Har- 

 vard University. 



This paper discussed two questions: (1) Are 

 hanging tributary valleys a reliable indication of 

 glacial erosion of the main valley? (2) May not 

 hanging tributary valleys result from glacial 

 %cidening of the main valley, instead of from 

 glacial deepening f It was shown that while the 

 formation of hanging valleys by normal stream 



