118 NEW YORK STATE MUSEUM 



evenness of grain, do not weather into blocks, are often schistose 

 and usually show the red tinge due to the presence of garnet, 

 which seldom appears in the diabase. 



An interesting feature of these dikes, which has been empha- 

 sized in previous reports, concerns their distribution. In Clinton 

 and Essex counties they are very numerous, so much so as to form 

 in places quite a respectable proportion of the whole rock mass. 

 In Franklin and Hamilton they are much less abundant, and in 

 Franklin at least they are mostly in its eastern half. In the west- 

 ern Adirondacks they are practically absent. 



They are also more abundant in the north than in the south- 

 east, more being found to the unit of area in Clinton than in 

 Essex. In other words, they are most concentrated toward the 

 northeast, so that the main center of the igneous activity must 

 have been in that region. Close to the Potsdam boundary in 

 Clinton they make a very impressive display. They may be even 

 more abundant beneath the cover of the later rocks to the north- 

 ward. 



Along with the diabases, though much fewer in number, are 

 complementary acid dikes which may be classed as a whole as 

 syenite porphyries. These show the same center of activity 

 though not spreading so far away from it. They are practically 

 confined to Clinton county and are most abundant near the Pots- 

 dam boundary. They range from red to brownish black in color, 

 average wider than the diabase dikes and are more apt to con- 

 tain large, porphyritic crystals. 



In Franklin county 37 dikes have been noted to date, though 

 these can be but a small proportion of the whole number. Their 

 situation is indicated on the accompanying map. Two are of 

 syenite porphyry, the only two dikes of this rock so far met with 

 in the whole Adirondack region outside of Clinton county. The 

 others are all diabases. Several of these hold bronzite, not a 

 common mineral in the Adirondack diabases which have hitherto 

 been described. There are several types present, as already 

 stated, and they merit a more detailed description than they have 

 yet received. 



