324 PROCEEDINGS OF THE ACADEMY OF [1892. 



October 4. 

 Mr. Charles Morris in the chair. 



Twenty-eight persons present. 



Papers under the following titles were presented for publication: — 



"The Batrachia and Reptilia of North Western Texas," by E. D. 

 Cope. 



" On a Collection of Batrachia and Reptilia from Washington and 

 British Columbia," by E. D. Cope. 



" Notes on a Collection of Shells from the State of Tabasco, 

 Mexico," by Henry A. Pilsbry. 



Geology of the Isles of Shoals. — Mr. Theodore D. Rand 

 remarked that Hitchcock, in his Geology of New Hampshire, 

 devotes but a few lines to these islands, stating that their geology 

 has been neglected. 



They are evidently the remains of a single island eroded by the 

 Atlantic Ocean and are composed of gneissoid rocks with a number 

 of trap dykes. 



The rock is chiefly a coarse orthoclase-muscovite gneiss, of which 

 the orthoclase constitutes probably eighty per cent, the quartz less 

 than twenty and the muscovite probably not over two per cent. 

 Inter-stratified in this coarse gneiss is a fine-grained variety contain- 

 ing much more mica and usually of a dark gray color. In some 

 places this contains serpentine veins of orthoclase, in this as well as 

 in otlier respects resembling our Manayunk schists and gneisses. 

 Garnets, while not entirely absent, are quite rare, and the rock very 

 rarely apj^roaches a schist. The strike is pretty uniform, about N. 

 70 E., while the dip varies, though usually 70° to 90° N. W. 

 Through these rocks pass numerous joints, many of them very 

 irregular. Along these joint-planes erosion has taken place leav- 

 ing a very rough and irregular surface, the remaining rock being 

 hard and not much disintegrated. 



Crossing the islands in a general northeast and southwest direc- 

 tion are trap dykes of varying width, from one to ten feet. These 

 form special lines of erosion, and are invariably lower than the 

 adjacent gneiss, though apparently much harder. All exposed 

 masses seem fresh and undecom posed. 



Most remarkable among these is one at the southeast end of Star 

 Island. It is about six feet across and extends at an acute angle 

 from the south to the east shore. Its strike is N. 35 E., its dip 85° 

 to 90° N. W., with two sets of joints, one parallel to the dip, the 

 other nearly coincident with the stratification of the adjacent gneiss. 

 At each end the dyke was deeply eroded and the adjacent gneiss in 



