12 



Report of the State Geologist. 



Smyth.* Similar gabbros have an extended distribution among the older rocks 

 to the south and west. 



The wide mineralogic variations commonly exhibited by these rocks find 

 excellent illustration here, the typical gabbros grading into diorites on the one 

 hand, and into norites on the other. All are more or less foliated, the horn- 

 blendic Varieties most markedly so. 



Along with these rocks may well be included the narrow, dike-like bands of 

 hornblende-plagioclase gneiss found everywhere cutting the older gneisses of 

 the region, and which apparently represent ancient diabase or diorite dikes. 



V. Granite. Granitic rocks are frequent in the Adirondacks. In part 

 they represent merely granitic phases of the basal gneisses, but in part they 

 are of later date. The -writer has frequently found in the northern Adiron- 

 dacks, granites which cut across the gneisses, and Smyth has described an 

 occurrence from St. Lawrence county with irruptive contact against Grenville 

 limestone.f Evidence of the time relation between the granite and the gabbros 

 is not at hand, though Smyth has described a contact where he believes the 

 latter to be the younger. J 



The granite is commonly of red color, well jointed, unlike the gneisses, 

 and composed essentially of quartz, orthoclase, microcline and oligoclase, ferro- 

 magnesian silicates being absent, or at best only sparingly present. 



VI. Dynamic Metamorphism of the Region. After the time of these 

 various igneous intrusions the region was subjected to intense dynamic 

 metamorphism, whereby secondary structures were produced and the primary 

 ones destroyed ; the rocks were all rendered thoroughly crystalline and their 

 original relationships masked. That the rocks now exposed at the surface 

 were then deeply buried beneath other rocks, since removed by erosion, is 

 shown by the manner in which the rocks adjusted themselves to the forces 

 acting upon them, the adjustments being of the kind that can only occur under 

 heavy load. 



VII. Pre-cambrian Dikes. At some period subsequent to the meta- 

 morphism of the region, all the rocks so far described were fissured, and 

 through the fissures thus formed fused rock made its way toward the surface. 

 Along some of these fissures faulting took place. That earth movements pro- 

 duced results of this character leads to the belief that the rocks were under 



*C. H. Smyth, Jr., Bull. G. S. A., Vol. VI, pp. 269-274. Also, Am. Jour. Sci., Vol. XLVIII, pp. 5I-C3 and Vol. L, 

 pp. 273-281. 



t C. H. Smyth. Jr. Bull. G. S. A., Vol. VI, p. 266. 

 X C. H. Smyth, Jr. Bull. G S. A., Vol. VI, p. 270. 



