GEOLOGY OF THE LONG LAKE QUADRANGLE 473 



Gabhro border of the anorthosite. The gradual change in com- 

 position outlined above continues until, as an extreme product, a 

 rock is obtained in which the heavy, dark colored minerals equal 

 or exceed the feldspar in amount. A steady diminution in coarse- 

 ness of texture accompanies this change, the uncrushed feldspar 

 cores become continually smaller and less numerous till they 

 finally disappear, and foliation becomes more and more promi- 

 nent, so that the final product of the change is a heavy, dark col- 

 ored gneiss which, bears no resemblance whatever to the normal 

 anorthosite, and would not be recognized as a variant of that 

 rock by an observer who met it for the first time, coming upon it 

 from without the anorthosite area. If approached from the other 

 side however the steady change from one rock into the other is 

 plainly manifest. This final rock is not anorthosite but gabbro, 

 and the rock intermediate between the two may be called anortho- 

 site gabbro. It is perfectly evident from the field relations that 

 this border phase of the anorthosite was produced by some proc- 

 ess of differentiation in the general mass of igneous material 

 after it had reached its present resting place, prior to or during 

 solidification. 



For the larger part of its extent across the quadrangle the syenite, 

 to be next described, adjoins the anorthosite, and a moment's in- 

 spection of the geologic map makes it evident that it has encroached 

 upon, and cut out the anorthosite border to some extent. Between 

 Follensby pond and the Raquette is a considerable mass of syenite 

 which cuts out nearly the whole of the gabbroic border locally, 

 and holds great inclusions of it likewise. In the Tupper Lake vicin- 

 ity also the anorthosite has been badly cut out by syenite, and 

 here again much of the gabbro border has disappeared. Outside 

 the limits of the quadrangle there are localities where the gabbro 

 border is lacking where its nonappearance is clearly owing to 

 faulting. It is quite likely that originally the entire anorthosite 

 area was characterized by such a differentiation border. 



In addition to this border differentiation to gabbro, a similar 

 change has also taken place here and there within the general anor- 

 thosite mass. A prominent area of the sort occurs near Panther pond, 

 within a mile of Upper Saranac lake, which has been mapped as 

 gabbro. As it is approached the anorthosite becomes rapidly more 

 gabbroic, but at the same time dikes of gabbro appear, cutting 

 the anorthosite and in regard to the gabbro center it is not certain 



