GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



305 



The other minerals call for no special comment. An augite 

 which is light green in thin section, is next in abundance to the 

 feldspar. Orthorhombic pyroxene is in general not so prominent. 

 It is usually hypersthene but sometimes bronzite. Ilmenite 

 always occurs with these, but in the normal anorthosite all these 

 are in small quantity, constituting ordinarily less than 5# of the 

 rock. 



Texture. The original anorthosite must have been extremely 

 coarsely crystalline, and likely coarsely porphyritic. Under meta- 

 morphism the rock has been granulated in varying degree, here 

 but little, leaving the rock still 

 very coarsely crystalline, there 

 excessively, producing a finely 

 sranular rock, all intermediate 

 gradations between the two be- 

 ing found. In the coarser rocks 

 the large feldspars are often 

 from 2 to 5 inches in length and 

 are universally dark colored, 

 often showing straining and 

 bending as a result of meta- 

 morphism. The granular feld- 

 spar is lighter colored and in 

 thin section does not show the 

 opaque rods which characterize 

 the other. It has plainly origi- 

 nated from the crushing, in 

 whole or part, of the large feld- 

 spars, crushing under such 

 great load of overlying materials as to cause the rock to remain 

 firm and resistant during the entire process; in fact, the granu- 

 lated rock is often stronger than the other. 1 



Differentiation. As the borders of the anorthosite are ap- 

 proached, the rock invariably shows some variation in character, 



1 For detailed descriptions of these textures, .see Adams. F. D. Geol. Sur. 

 Can. An. Rep't. v. 8. pt J. p. 103-15. 



Fig-. 1 Drawing- of a portion of a hand 

 specimen of anorthosite gabbro from near 

 Keeseville, showing a large labradorite 

 crystal which has been broken into several 

 pieces, these being forced apart and granu- 

 lnted around their edges and the gaps 

 tilled by granulated material, the rock 

 remaining hrm and tough throughout. 

 The black mineral is ilmenite, and the 

 dotted is granular garnet. The uniform 

 cleavage and twinning of the fragments 

 of the large crystal demonstrate then- 

 identity. 



