616 WILLIAM J. MILLER 



in the rock, resulting from pressure which acted subsequent to 

 or possibly during the last stages of its consolidation." 1 



FOLIATION OF THE GABBRO 



The gabbro here considered is the latest Adirondack intrusive 

 which exhibits foliation and granulation. Diabase is the only 

 intrusive still younger. A few years ago the writer 2 discussed the 

 origin of certain primary variations of Adirondack gabbro. At that 

 time, in accordance with the usual idea, the foliation was thought 

 to be largely a secondary structure and so was omitted from the 

 discussion. 



Character of the gabbro. — Most of the gabbro is in the form of 

 small stocks or bosses, the outcropping areas typically ranging from 

 elliptical to almost circular, and the dimensions from a few rods 

 to one or two miles. They are especially well-shown on the North 

 Creek, Long Lake, Elizabethtown, and forthcoming Blue Mountain 

 geologic maps. Most of them are of pluglike or pipelike form, with 

 practically vertical, sharp contacts against the country rock. The 

 stocks exhibit many variations in composition and texture from the 

 normal, homogeneous, dark, basic gabbro with diabasic texture, 

 to lighter- colored rocks of dioritic and even syenitic make-up. 

 They also range from fine-grained to very coarse-grained with feld- 

 spars up to an inch or more in length. The typical gabbro con- 

 tains principally basic plagioclase, pyroxene, hornblende, biotite, 

 garnet, and ilmenite, while orthoclase and quartz often occur 

 in the more acidic facies. 



A very important feature, from the standpoint of our present 

 discussion, is the almost universal development of highly foliated 

 amphibolitic borders which often completely surround the stocks, 

 while the interior portions of the typical stocks are usually non- 

 foliated. In many cases, however, stocks seem to be wholly 

 changed to amphibolite, or only very small cores remain. In 

 still other cases coarse-grained gabbro shows gneissoid structure 

 thoroughly developed throughout. As a rule the gabbro exhibits 

 as good, :f not better, foliation than the older intrusives. Often 



1 F. D. Adams, Geol. Surv. Can., Guide Book No. j, 1913, p. 17. 

 J W. J. Miller, Jour. Geol., XXI (1913), 160-80. 



