MARYLAND GEOLOGICAL SURVEY 127 



The mineral of this outer border possesses all the characteristics of 

 adjacent, easily determined hornblende, except that of the hornblende 

 cleavage, which the form of the mineral renders impossible. There 

 seems little doubt that the outer border is hornblende. On its inner 

 edge it becomes colorless and resembles the transparent constituents 

 of the inner zone. This inner zone is probably tremolite. The altera- 

 tion of olivine to tremolite and actinolitic hornblende is not uncom- 

 mon and has been frequently described. It has been considered a 

 magmatic reaction rather than the result of dynamic metamorphism, 

 subsequent to the consolidation of the rock. There seems to be no 

 convincing proof on this point. 



The pyroxene of this olivine-gabbro is partially altered to horn- 

 blende, the result of a reaction between the ferromagnesian silicate 

 and the lime-silicate. The feldspar shows some alteration to epidote, 

 but is, for the most part, quite fresh. 



The alteration of the norites and hypersthene-gabbro has resulted 

 in the formation of a hornblende-saussurite rock, producing in the 

 hand specimen a mottled white and green aspect. Rarely has the 

 rock been subjected to such pressure as to produce a schistose struc- 

 ture. In this case chlorite has been largely developed as well as 

 uralite. Such schistose gabbro is exposed on the right bank of the 

 Octoraro at the bridge, one mile south of the State boundary. 



The following analysis represents a single specimen of the norite. 

 It will be seen that the feldspar is even more basic than is usual for 

 this type. The slide shows the emergence of an axis on 010, with an 

 extinction of — 37°, and an extinction on 001 of — 33°, — 36°, 

 — 37°. The feldspar was accordingly given the composition of 

 Abi An 12 . With this composition for the feldspar, the alumina and 

 lime percentages accord, and the above results are obtained by 

 recalculation. 



The slide also shows an alteration of the hypersthene to hornblende. 

 This change always takes place at the expense of the feldspar from 

 which the lime is obtained. 



