[6 NEW YORK STATE MUSEUM 



Within the belt are comprised several large magnetite deposits, 

 including those at Lyon Mountain and vicinity, the Arnold hill 

 and Palmer hill bodies, and a number of smaller ones. Oppor- 

 tunity has been afforded the writer of studying the gneisses in the 

 field as well as to compare them with the rocks of other mining 

 districts. At many localities within the belt have been found 

 undoubted representatives of the igneous rocks. The acid gneisses 

 particularly contain cores which are coarsely textured, even por- 

 phyritic, and in other respects are analogous to the characteristic 

 Adirondack granites; the coarse phases can be traced at times by 

 gradation into fine grained gneissoid rocks which are evidently 

 only crushed portions of the same mass. It seems probable that 

 the granitic series will be found to abound throughout, the belt, 

 yet there are large areas of gneiss that can not be satisfactorily 

 correlated on the basis of present knowledge. 



Igneous intrusions. The plutonic igneous rocks of the Adi- 

 rondacks can be divided into four great groups, viz: anorthosite, 

 gabbro, syenite and granite. In their normal development the 

 individual groups are well contrasted by their physical appear- 

 ance, as well as by the peculiarities of their chemical and mineral 

 composition. They are all connected, however, by a series of 

 intermediate x rock types, presenting a variation scarcely inter- 

 rupted from the acid to the basic members. This close relation 

 between the groups is generally recognized to be an original feature, 

 due to a common derivation from a continuous magma in the 

 interior. By repeated segmentation and intrusion the magma 

 has given rise to the rock series now existing at the surface. 



Anorthosite. The anorthosite is the earliest in point of time of 

 the intrusions mentioned. Its occurrence in the Adirondacks was 

 made known by Professor Emmons who described it under the 

 name of hypersthene rock. That he recognized its igneous nature 

 is clearly evidenced by the fact that in his report it is placed among 

 the unstratified class of rocks, though the name he used has given 

 way to the more appropriate one which emphasizes the feldspathic 

 component. Hypersthene plays a very subordinate role in the 

 composition of the Adirondack anorthosite. The rock forms the 

 central massive of highest uplifts. It is exposed over an area that 

 is roughly triangular in shape with its base on the north along the 

 Essex-Clinton county border, extending west from Lake Cham- 

 plain for over 50 miles, and its apex in southern Essex county 

 near the Warren county line. The area probably exceeds 1200 

 square miles. There are some belts of gneisses and erystalline 



