484 NEW YORK STATE MUSEUM 



discriminate between the two gabbros on the basis of difference 

 in character. 



Diabase. But two dikes of this rock have been noted within the 

 quadrangle limits. The larger and more accessible of the two is on 

 Round island, in Long lake, showing the usual dense, heavy; black 

 rock with chilled borders and coarser center, and with occasional 

 porphyritic feldspars. On the southeast edge of the island both 

 contacts with Grenville rusty gneisses are exposed, showing the 

 dike to be 30 feet wide and to bear n. 30 e., and that it is not 

 vertical but dips 6o° s. No fresh material could be obtained from 

 the dike but it is one of the ordinary olivine diabases of the region. 



The other dike was noted on the south slope of an anorthosite 

 hill on the east edge of the quadrangle. Neither contact showed- 

 and there was but a single exposure so that its thickness and trend 

 can not be stated. It is an ordinary nonporphyritic diabase. Two 

 other similar dikes are known just west of the quadrangle limits on 

 the Tupper lake sheet, and a few have been located on the compara- 

 tively unexplored and rugged Santanoni quadrangle, just east. 

 They are infrequent in the mid- Adirondack region, though abund- 

 ant farther east. 



ROCK STRUCTURES 



Foliation. x The rocks of the northern half of the quadrangle are 

 chiefly massive eruptive rocks, in which foliation is absent, or at 

 best only rudely developed. This is mainly owing to their highly 

 feldspathic character, and the scarcity of the minerals which are 

 good producers of foliation. The gabbroic anorthosite and the basic 

 syenite have it much better developed, and the tendency of the 

 quartz to assume the leaf type is responsible for a poor foliation in 

 some of the granites. Conspicuous foliation is only found in the 

 Grenville rocks and portions of the Long lake, and Grampus 

 gneisses. 



In the Grenville sediments foliation and bedding correspond, in 

 all cases in which it has been possible to determine their relation- 

 ship. But the Grenville rocks cover such a comparatively small 

 portion of the area, exposures are so infrequent and in general so poor, 

 and the stratigraphy of the series is so little known, that scant idea 

 of the general structure could be obtained from the usual methods. 



1 Foliation is a convenient term for that variety of flow cleavage found in wholly crystal- 

 line rocks, which have wholly or largely recrystallized under pressure, and which hence 

 possess a parallel arrangement of mineral particles, resulting in a capacity to split more 

 readily in one direction than in any other. 



