J. F. Kemp — Great Shear-zone in the Adirondacks. 113 



The strip of the shear-zone may have been more basic origi- 

 nally than the walls, and may have contained more bisilicates 

 or else infiltration has rendered it more basic. Alternations 

 from light to dark so-called norite are not uncommon in the 

 mountains, especially where the rocks are gneissoid. Being 

 dynamically affected by faulting its minerals were crushed to 

 the shreds in which we now see them and the secondary ones 

 resulted. The hypersthene is regarded as an original mineral 

 because it is abundant in the less disturbed districts and is a 

 more distinctively igneous mineral than those derived from it. 

 It cannot be denied, however, that the sheared zone may have 

 been an original dike. If so its mineralogy and structure have 

 been completely reorganized. So far as observations go, dia- 

 base dikes are quite plentiful in the outskirts of the mountains, 

 but in neither of these characters does the shear-zone bear 

 indication of ever having been diabase. The dynamic meta- 

 morphism of diabase has been shown in numerous instances in 

 this country and abroad to yield amphibolites. The shear-zone 

 is more like an eklogite than anything else. Moreover an 

 intrusion of norite as a dike is a new phenomenon for this 

 region whose noritic exposures are singular massive or gneis- 

 soid and send out so far as known, no dikes whatever. It is 

 stated by Redfield that other and smaller dikes similar to the 

 large one occur on Mt. Mclntyre and frequently appear in the 

 beds of brooks. He refers the lines of drainage to the greater 

 relative decomposition of the supposed dikes. While no other 

 one was seen by the writer which would be analogous to the 

 shear zone, these smaller occurrences ought to prove to be 

 diabase or some similar rock if they really are intrusions. 



A mineral aggregate similar to that of the shear-zone was met 

 on Trembleau Point just south of Port Kent, and in the rail- 

 ways cuts. The country rock, which contains coarse plagio- 

 clase and monoclinic pyroxene as well as hypersthene, passes at 

 times into dark, gneissoid bands, easily mistaken at first glance 

 for trap which also seams the cuts in all directions. In the 

 slides these bands are practically identical with the Avalanche 

 shear-zone although at times containing more biotite. They 

 make no recesses because freshly exposed. 



A case of a shear-zone was also met at Hammondville in 

 the !No. 7 slope of the iron mines. The workings run down 

 following a bed of magnetite, for 700-800 feet. They pene- 

 trate three narrow diabase dikes, by each of which the ore is 

 faulted about 15 feet. The workings finally met another dark 

 strip and the ore was cut off. A specimen was gathered under 

 the impression that it was trap, but the section shows crushed 

 remnants of the gneiss which is the wall rock. It is mostly' 

 chloritic alteration products, with shattered and strained grains 



