58 G. H. Smyth, Jr. — Gcibbros in the Adirondack Region. 



diallage are lacking, the pyroxene being of augitic habit. The 

 angle of extinction is about 40°. This pyroxene is often in 

 somewhat larger grains than the hypersthene. 



The hornblende, as already stated, is extremely variable in 

 quantity, but is never entirely absent. It is compact, brown 

 and strongly pleochroic ; a light yellow, b deep brown, c dark 

 greenish-brown, with the absorption c>b>ci. It often contains 

 small black inclusions, looking like magnetite. It is of con- 

 siderable importance in making out the history of the gabbros 

 and their relations to other rocks of the region to ascertain 

 whether the hornblende is original or paramorphic. In favor 

 of the latter supposition is the fact that the hornblende often 

 partly encloses grains of pyroxene into which it seems to pass 

 gradually. This intimate association is very common and 

 often extremely marked. Paramorphism would, moreover, 

 readily explain the increase of hornblende in some portions of 

 the rock, even amounting to complete exclusion of pyroxene. 

 On the other hand, the association of the hornblende and 

 pyroxene clearly is often, if not always, the result of parallel 

 growth and accidental juxtaposition. Further, in many cases 

 the apparent gradual transition between the two is shown by 

 careful observation to result from an approach to parallelism 

 between the plane of their contact and the plane of the sec- 

 tion. When this is not the case the line of junction of the 

 two minerals is generally distinct and shows no indication of 

 interpenetration. Then, too, the amount of hornblende in 

 the rocks shows no close relation to the intensity of the me- 

 chanical force to which they have been subjected, as it is just 

 as likely to be abundant in a massive rock as in one that is 

 prominently gneissoid. Taking these points into consideration 

 it seems probable that the hornblende is an original constituent 

 of the gabbro and that the hornblendic phases of the rock are 

 the result of local variations in the original magma, or of 

 differences in the conditions of solidification. 



Biotite is of very minor importance as a constituent of the 

 gabbro, and need receive no special description. Its distribu- 

 tion is, however, of interest. Almost without exception in 

 the finer gabbros the biotite occurs in close proximity to the 

 plane of contact with the gneiss. The hornblende shows a 

 tendency in this direction, but nothing like that of the biotite, 

 which also tends to arrange itself parallel to the contact. The 

 exceptions to this rule are few. 



Magnetite is an unfailing and important constituent. It 

 forms large irregular grains, plainly primary and in consider- 

 able quantity. Tested with H 2 2 it shows the presence of 

 some titanium, but as it is readily soluble in acids and strongly 

 attracted by the magnet it must be a titaniferous magnetite 

 rather than ilmenite. 



