278 PROCEEDINGS OF THE ACADEMY OF [1880. 



This forination, so abundant in Delaware, is thus proved to be 

 by no means a local one, and it is probable that it will be identified 

 with some of the formations grouped together under the name of 

 " Southern Drift." 



The Bryu Mawr gravel has also recently been found in the 

 Montgomer}' County limestone valley, and there seems to be a 

 close connection between it and the surface or drift iron ores of 

 that valley. Some of these ores appear to be simply a verj' fer- 

 ruginous variety of the Mt. Holly conglomerate. They overlie 

 unconformably the steeply-dipping decomposed shales which hold 

 a more ancient and richer ore. 



In Bucks Count}' there occurs a gravel different from any yet 

 described, M^hich at first occasioned some confusion. It has proved 

 to be the result of the decomposition of the lower Triassic con- 

 glomerate, the pebbles of which, loosened from their cementing 

 material, have been scattered tlirough the soil. These Triassic 

 pebbles are formed of gneiss, not Potsdam. Hills of red shale 

 border this gravel. 



A preliminary map of the Surface Geology of Southeastern 

 Pennsylvania was exhibited, and it was suggested that its publica- 

 tion would be of service to man 3^ besides geologists. 



April 28, 1879. 



On some Enclosures in 3Iica. — Mr. Lewis exhibited some plates 

 of Muscovite which he had found on Shoemaker's Lane, German- 

 town, which contained microscopic ciystals of peculiar shape. 

 They consisted of a dark green mica, probably Lepidomelane, 

 in minute sharp crj^stals thickly- disposed throughout the 

 muscovite. These crystals were frequently arrow-shaped, and 

 generally much elongated. Large numbers of them were shaped 

 like a musket. They were very different from any of the enclo- 

 sures in the muscovite of Pennsbury, Del. Co., and were interesting- 

 objects under the microscope. 



On Dendrites. — Mr. Henry Carvill Lewis made some obser- 

 vations upon dendrites and their mode of growth. He stated that 

 dendrites were not caused by filtration of metaliferous water, but 

 that they frequently grow upward by chemical or capillary action. 

 He described an exposure of white lower Triassic sandstone in a 

 quarry in the southern part of Noriistown, where dendrites of 

 oxide of manganese were seen upon the surface of the rock, 

 growing from below upwards. The dendrites were apparently in 

 process of growth, and were so soft that they could be scraped 

 with a knife from the rock. The material thus obtained gave a 

 bright metallic streak on the fingers, and was shown by the blow- 

 pipe to be hydrous oxide of manganese. It was otaserved that 

 while the rock above and below these dendrites was spotted with 

 minute rust-specks of manganese, the i)ortion upon which the 



