56 



Composition in 100 parts. 



Silica 



Alumina 



Peroxide of Iron 



Magnesia . 



Lime 



Soda . . . 



Potassa 



G6.51 



17.67 



1.33 



0.30 



1.24 

 3.03 

 9.81 



8.66 



Oxygen. 



34.55 



8.25 



0.41 



o.ne^ 



0.347 ! 

 0.774 f ^* y97 

 1.660 J 



11.56 



99.89 



In comparing this result with the two former, Mr. Boye observed, 

 that this felspar approaches, in composition, more nearly to the first 

 of the others ; but, that a small portion of the potassa is replaced by 

 lime, to which latter, and the oxide of iron, it probably owes its higher 

 specific gravity. To these two ingredients, as well as to the full pro- 

 portion of silica which it exhibits, (3x11.56 = 34.68,) may be refer- 

 red the indestructibility and other superior qualities which are as- 

 cribed to this variety. 



Mr. B. closed his remarks by mentioning the recently published 

 report of Professor Booth, on the geology of Delaware, as presenting 

 a detailed and interesting notice of these several rocks. 



Mr. Lea submitted to the examination of the members, a 

 volume containing two hundred specimens of photogenic draw- 

 ings of the plants in the vicinity of Philadelphia, by his son, 

 Mr. Carey Lea. 



Prof. A. D. Bache laid before the Society a diagram repre- 

 senting the direction and force of the wind, and the amount 

 and rate of fall of the rain, during the severe gust of April 2d. 



The diagrams were copies of the register kept at the Philadelphia 

 Magnetic Observatory, at the Girard College, traced by the self- 

 registering anemometer. The bi-hourly register of the barometer 

 showed, that the pressure diminished from early in the morning to 

 the time of the observation next preceding the gust, at the rate of 

 about .09 of an inch every two hours. The wind during the day 

 was generally from the S., and light, veering in the afternoon, occa- 

 sionally nearly to S. by W. Between 6h 7m, P. M. and 6h 20m, it 

 had changed from S. to N. W. by W., and the pressure between 6h 

 14m and 6h 21m increased from to 20 lbs. avoirdupois, upon the 

 square foot. Rain began to fall about 6/i 15m, the wind having at 

 that time reached the W. in direction, with a force of less than ^ of 



