478 H. p. GUSHING ASYMMETRIC DIFFERENTIATION IN SYENITE 



asymmetry is thought to be owing to incorporation and assimilation of 

 material from the bordering rocks. 



General Description 



The syenite rock in question is a common one in the Precambric area 

 of northern New York, occurring in both large and small masses, in 

 bathyliths and stocks, in dikes and sills. It is a member of the series 

 of great intrusive bodies which invaded the region in pre-Cambrian time, 

 and which solidified far below the surface, and is younger than the 

 Grenville sediments and the Laurentian granite-gueisses of the district. 

 Of the post-Laurentian intrusives it was the second in order of appearance, 

 being younger than the anorthosite. As the above form the bulk of the 

 Precambrian, it follows that the usual contacts of the syenite are those in 

 intrusion. The rock is usually gneissoid, the smaller masses especially 

 being thoroughly so, and so stretched and interleaved with other gneisses 

 that the relations are much disguised. The especial mass under consider- 

 ation is one of the most extensive, is adjoined by and shows contacts 

 against two of the three great groups of older rocks, and the meta- 

 morphism is not sufficiently pronounced to obscure its relations to them. 

 It therefore furnishes a very satisfactory sample for the study of these 

 relations. Its main disadvantage is that it lies in a forested region, in 

 which outcrops are not as abundant nor as satisfactory as could be wished 

 and in which getting about is slow and laborious work. 



This bathylith is situated near the central part of the woods, in the 

 district centering about Big Tupper lake, and its main portion lies within, 

 the limits of the Long Lake topographic sheet, though running well 

 into the Tupper Lake quadrangle, next west. It is of rudely elliptical 

 shape, with a minor axis of about 8 and a major axis of from 13 to 15 

 miles; hence with an area of some 75 square miles as cut by the present 

 erosion surface. On the north and east it is adjoined by anorthosite 

 and on the south and west by Laurentian granite-gneiss containing much 

 included amphibolite. Against both these it shows irruptive contacts, 

 it incloses numerous fragments of them, and it sends out abundant dikes 

 into them. 



The normal Syenite 



When fresh this is a green to grayish green rock, which experiences 

 rapid color change upon exposure to the air, becoming first a deeper 

 green, then more slowly passing to a yellow brown, and finally to a deeper 

 rusty brown.* The color change does not at all affect the freshness of 

 the rock as seen in thin-section. 



• R. A. Daly : Bull. no. 209, U. S. Geological Survey, pp. 51-53. 



