Frazer.] 



402 



[April 16, 



ON THE TRAPS OF THE MESOZOIC SANDSTONE IN YORK 

 AND ADAMS COUNTIES, PENNSYLVANIA. 



By Persifor Frazer, Jr., A. M. 



{Bead before the American PhilosopJiical Society, April 16, 1875.) 

 Chemical Properties. 



All igneous rocks consist principally of compounds of some kind of 

 feldspar (or Nepheline or Leucite) with pyroxene, hornblende, mica or 

 quartz, and generally with some magnetite and other subordinate min- 

 erals. All these again may be divided into those poor in Silica or Basic, 

 or those rich in Silica or Acidic* 



The average compositions of these two kinds of igneous rocks are : 



Taking tliese ideal average percentages of the constituent compounds 

 of these two classes of rocks, throwing them into a more convenient 

 form and neglecting small fractions, we have : 



BASIC. 



PER CENT. 



Silicon 24.96 



Aluminum 9.00 



Iron from Fe.^g say 4.5 



Iron from FeO say 5.0 



Calcium 5.7 



Magnesium 4.3 



Potassium 4.1 



Sodium 2.9 



Hydrogen , 0. 2 



Total oxygen 47.1 



Of course it is understood that these figures represent no combination 



* Eocks Olassified and Described, by B. v. Cotta. Translated by H. Lawrence. Lon- 

 don. Longmans, G-reen & Co., 1866. 



