GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



401 



the rock. In all the large igneous masses, the general more basic 

 character of their peripheral portions has resulted in the forma- 

 tion of such minerals there, hence their tendency to pass over into 

 gneisses at their borders, a tendency so widespread as to be prac- 

 tically universal. The gabbros of the region possess throughout 

 a large proportion of such minerals, and in the writer's experience 

 they, though the youngest of the igneous rocks of the group, are 

 much more uniformly gneissoid than are any of the others. True, 

 comparatively unchanged cores remain in nearly every case, so 

 that the original character of the rock may be demonstrated, but 

 this is usually of small bulk in comparison with the hornblende 

 gneiss, produced from it by metamorphism. 



In many of the granites also there is a great scarcity of the 

 foliation-producing minerals, the rock being mainly, or wholly, 

 constituted of quartz and alkali feldspars. These rocks are 

 apt to lack foliation, and then not infrequently have a somewhat 

 similar linear structure, the quartzes being drawn out into 

 spindles and pencils, with a direction corresponding to the folia- 

 tion direction of the inclosing rocks. A similar tendency may 

 often be noted in the more quartzose syenites. This structure 

 has only been noted in these quartzose rocks, hence a natural 

 tendency to attribute it to the mineral composition. But 

 coupled with that may well have been such slight differences in 

 the mean and maximum pressures in the rock that it suffered 

 nearly equal shortening in two directions at right angles, and 

 elongation merely in the third. 1 



The foliation in the Precambric sediments seems, in general, 

 to be parallel to the bedding, so far as the latter may be made 

 out. Over the greater part of the district the dips are compara- 

 tively steep, ranging in general from 20° upward. Judging 

 from the writer's own experience, and from the published data 

 of other observers, the strike is seldom uniform over any con- 

 siderable area, but is now to the northeast, now to the north- 

 west. Xorth and south, or east and west 'directions are much 

 more infrequent. The shifting of the strike direction indicates 

 that we are dealing with folded rocks, and that this is actually 

 the case is readily demonstrated in the Grenville sediments, 

 but with difficulty elsewhere. It is also evident that the folds 



Joskins, L. M. U. S. Geol. Sur. 16th An. Rep't, p.870. 



