GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



315 



the ordinary rocks the pyroxenes much predominate, but these 

 shade into rocks in which the reverse is true, the hornblende in- 

 creasing up to complete exclusion of the pyroxenes. With this 

 increase in hornblende biotite always appears in the rock, this 

 mineral being usually lacking in the pyroxenic varieties. Further, 

 the more quartzose varieties are more apt to be those with pre- 

 dominating hornblende, though this is by no means a general rule. 



Extreme variations. Besides these minor changes in character, 

 more extreme variations of these rocks occur, on the one hand 

 into granitic, on the other into gabbroic rocks, variations which 

 can however be traced into the ordinary rock step by step. 



The most striking instance of a change of this sort which has 

 received careful description is found in Smyth's account of the 

 Diana syenite belt. 1 At the time when this paper was written 

 the syenites had not been differentiated from the anorthosites and 

 gabbros, and the rock was described as a variety of gabbro, and 

 its variations as variations of a gabbro mass. Smyth's descrip- 

 tion, however, shows that he clearly apprehended the differences 

 between the rock and ordinary gabbro, and he distinctly states its 

 syenitic character, and moreover in a later publication wholly 

 withdraws the rock from the gabbro class. 2 Unfortunately this 

 has not been apprehended by the several writers who have had 

 occasion to refer to this important paper, and the special varia- 

 tion into a red gneiss w hich will be shortly described is referred 

 to as a variation of gabbro into red gneiss. The truth is that 

 no such variation of gabbro is known in the region, while varia- 

 tions of the syenite into red. granitic gneisses are the rule rather 

 than the exception. 



The special interest attaching to this Diana syenite arises from 

 the clearly exhibited differentiation of the gray, feldspathic syen- 

 ite into a dark colored rock of gabbroic appearance. The miner- 

 als are the same in both and are practically the same as in the 

 Loon lake rock, but the pyroxene and hornblende are in much 

 larger quantity in the dark rock, constituting from one third to 

 one half of the whole, while they appear in but scant amount 

 in the ordinary syenite at Diana. At the same time plagioclase 

 increases in amount at the expense of the microperthite. and 



iGeol. Soc. Am. Bui. 6:271-83. 



2 N. Y. State Geol. 17th An. Rep't. p. 472. 



