C. H. Smyth, Jr. — Gabbros in the Adirondack Region. 59 



The only macroscopic structural variations in the gabbros 

 are differences in coarseness of grain and in degree of foliation. 

 The grain as a rule remains nearly constant throughout any 

 given body of the rock but varies somewhat in different 

 patches. The hornblendic varieties are apt to be coarser than 

 the normal rock, and as the hornblende is often more abundant 

 toward the margins of the rock mass, these portions are 

 sometimes decidedly coarser than the central portions, revers- 

 ing the general rule in regard to intrusive rocks. 



The gneissoid varieties when examined under the micro- 

 scope show the effects of pressure in a greater or less amount 

 of crushing of the minerals, besides the modulatory extinction, 

 bending of twinning lamellae, etc., which are seen in all sec- 

 tions. At the same time that the gneissoid structure becomes 

 marked, the structure of the hypersthene undergoes a con- 

 spicuous modification. The mineral not only becomes shat- 

 tered like the other constituents, but sends out long slender 

 tongues into the surrounding feldspar. These tongues range 

 from slight projections of the hypersthene to string-like ex- 

 tensions which radiate from a hypersthene core. In still more 

 extreme cases this core is lacking and the hypersthene forms 

 curious rosettes. It is often impossible to determine directly 

 the nature of the mineral in the tongues, but it may be safely 

 regarded as hypersthene as there is a complete gradation be- 

 tween them and the larger extensions plainly composed of this 

 mineral. The larger tongues of hypersthene usually form a 

 granophyric intergrowth with the feldspar presenting an 

 appearance somewhat like that described by Bayley* in the 

 augite of Lake Superior gabbros. It is impossible to deter- 



Fig. 1. — Gabbro. Large piece of hypersthene sending out tongues into the 

 surrounding feldspar. Diameter of field ,l 7 mm . 



Fig. 2. — Gabbro. Separate tongues of hypersthene in feldspar. Diam. of 

 field 0-4 ! ™. 



* W. S. Bayley. A Fibrous intergrowth of Augite and Plagioclase, resembling 

 a reaction rim, in a Minnesota Gabbro, this Journal, III", xliii, p. 515. 



