NEW f ORE STATE MUSEUM 



abundant, the dikes running in every direction, and often branch- 

 ing. 



Plates L8 and 1!) illustrate both cases. 



A somewhat peculiar feature of these exposures is the entire 

 absence of contact metamorphism. When an acid intrusive 4 cuts 

 a basic calcareous schist, contact minerals would be expected, 

 but none occur here. Even the small tourmalin zones, developed 

 elsewhere under what would seem much less favorable condi- 

 tions, are lacking. 



However, while contact zones fail, some interesting mineral 

 localities occur on the ridge. The pegmatitic aspect of some of 

 the dikes led the writer to think that somewhere in the schist 

 would be found the localities which furnish the well-known speci- 

 mens of flesh colored orthoclase and specular hematite which 

 are, in many mineral collections, labeled as coming from 

 Alexandria. Such proved to be the case, these minerals oc- 

 curring in very acid pegmatites, so acid indeed that sometimes 

 they are little more than quartz veins. The specimens are so 

 well known as to require no description. These mineral veins 

 cut directly across the foliation of the schist and are sharply 

 defined. Seen by themselves, with the predominance of quartz, 

 and the presence of some calcite, they would perhaps be taken 

 for purely aqueous veins. But in the presence of the granite 

 dikes, between which and the veins there are various inter- 

 mediate forms, it seems practically certain that these mineral 

 veins are igneous in origin, presumably representing a very late 

 stage of activity, in which vapors and hot solutions played the 

 chief part. 



Two dikes of diabase were also noted in this ridge, but they 

 have no special significance. 



A mile farther south is another ridge of schist similar to the 

 above and to be classed with it. But it is comparatively lacking 

 in intrusions of the granite-gneiss and presents nothing of par- 

 ticular interest. This absence of intrusions in the ridge may be 

 due to its being farther from the contact with the granite-gneiss, 

 but the surrounding Potsdam hides all evidence on this point. 



