130 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



The pyroxenic constituents are hypersthene and diallage. Both 

 varieties may be present, or either one to the exclusion of the other. 

 "While they are sometimes quite fresh, more generally some stage 

 of alteration is observable. The uniform alteration product is green 

 hornblende. In the case of orthorhombic pyroxene the alteration is 

 always peripheral, while with monoclinic pyroxene it takes place 

 within the body of the mineral as well as on the surface. 



This difference is due, of course, to the respective compositions 

 of the two pyroxenes. The change of hypersthene to hornblende 

 must be a reaction between the ferro-magnesian silicate and the sur- 

 rounding feldspar, which furnishes the lime for the hornblende. 



In the case of the augite, on the other hand, the process is a true 

 epimorphism, and the alteration to hornblende is also always a direct 

 one. The alteration of hypersthene to hornblende may be direct or 

 through the intervention of tremolite. This mineral forms an inner 

 colorless zone between the hypersthene and the hornblende. 



The pyroxene-hornblende epimorphism in the Iron Hill gabbros 

 has been accurately described by Professor Chester, 1 who reports the 

 process of change of hypersthene to hornblende as always an indirect 

 one. Additional material from this region shows that a direct change 

 from hypersthene to hornblende may take place without the inter- 

 vention of tremolite. 



This pyroxene-hornblende alteration may be seen in all stages of 

 progress. When the last stage is reached a hornblende-feldspar rock 

 results, which is a not infrequent type in this area. This facies 

 represents a true meta-gabbro rather than a hornblende-gabbro, in 

 which the hornblende is original. 



The character of the feldspar also distinctly separates these horn- 

 blendic facies from the diorite, with which they have ordinarily been 

 classed. 



Pyroxene varies in proportion from slightly less than 50 per cent, 

 to 100 per cent of the rock. In the latter case the rock becomes a 

 pyroxenite of the hypersthenite or websterite type. 



1 Bulletin U. S. Geol. Surv., No. 59, pp. 23-27. 



