MAPPING CRYSTALLINE ROCKS OF ADIRONDACK REGION 133 



tion, as they are connected by every intermediate stage, and it is 

 clear that the former are massive phases of a part, at least, of the 

 •gneissic complex. As, however, the age and origin of large areas 

 of the gneiss are as yet inferential, it seems advisable to indicate 

 the granites and syenites wherever possible, keeping constantly 

 in mind their close connection with the gneisses. 



No attempt has been made to trace in detail the boundary be- 

 tween the crystalline and paleozoic rocks, and only incidental 

 <data bearing on this point have been accumulated. 



The work of the season has almost completed the reconnais- 

 sance of the western half of the Adirondack region. The extreme 

 southern part of Hamilton county and the adjacent part of Herki- 

 mer county have not yet been examined, and the same is true 

 of the vicinity of Black lake in St Lawrence county, and some 

 other limited areas in the northern part of the region. As to the 

 first gap, that in Hamilton and Herkimer counties, it is practi- 

 cally certain that it is an area of gneiss with small limestone 

 patches. But in the north there is so much variation in the rocks 

 that decided changes may come in suddenly and unexpectedly. 

 To the southwest of Black lake, directly in the line of strike, lime- 

 stone, gneisses and granites occur in profusion, and, as limestone 

 is known to occur on the shore of the lake, there can be no doubt 

 that, in a general way, this small area agrees with the region ex- 

 amined about Rossie and Redwood, but as to details nothing is 

 jet known. 



From what has been said above, it is evident that the results of 

 the season's work are chiefly areal. Nevertheless, in four partic- 

 ulars some light is shed on genetic and structural problems. 



In the last report it was shown that the hypothetic gabbro core 

 of the Adirondacks, with its narrow fringe of gneisses having 

 ^uaquaversal dip, has no existence, but, instead, that the region 

 consists chiefly of gneisses, cut by immense gabbro intrusions on 

 the east. Still, as there stated, it had not been shown that there 

 might not be areas of gabbro of some magnitude in the western 

 part of the region. This point is now settled; and it is clear that 

 an the entire western half of the Adirondack region gabbro occurs 



