REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1900 r31 



General statement 

 Good exposures abound through most of the anorthosite dis- 

 trict. The New York Central and Hudson River railroad cuts 

 across it from Rollins pond to Saranac, affording numerous rock 

 cuts in the low ridges which protrude through the sands, and 

 which, except for an occasional small dike, show no other rock 

 than anorthosite. The frequent ridges along the shores of Upper 

 Saranac lake and the adjoining ponds show no other rock. The 

 shores and islands of Lower Saranac and Round lakes^ the ridges 

 along the river from the lake to the village, the entire mass of 

 Boot Bay mountain are of anorthosite exclusively except for a 

 small intrusion of the later (hyperite) gabbro at the upper end 

 of the lake. At Ampersand miountadn^ Stonycreek mountain, the 

 country around the St Regis lakes and thence down the river to 

 Brandon, St Regis mountain, everywhere in the district, the same 

 features prevail. The uniformity of the rock is surprising, such 

 variations as occur being mainly due to the amount of crushing 

 which it has undergone and not to changes in composition. 

 Occasionally the rock takes on a larger dark silicate content, 

 more nearly approaching a gabbro, but, instead of its doing this 

 as a whole, the change is local in its nature. There is however a 

 usual change to a more gabbrold character as the periphery of 

 the mass is approached^ the ferro-magnesian silicates increasing 

 considerably in amount, though seldom exceeding 15^ of the rock 

 and usually not constituting over lOfc. Even this change is not 

 universal throughout the mass, as quite typical anorthosite can 

 be found close up to the edge of the intrusion. But the gab- 

 broid phase far outweighs the other in this situation, while the 

 reverse is true in the heart of the mass. Along with this change 

 in amount of ferro-magnesian silicates goes also a change in the 

 character of the feldspar. It usually becomes more acid, and 

 quartz sometimes appears as well, but there is no regularity in 

 the change. The recapitulation of some detailed observations is 

 necessary in order to reenforce these general statements. 



