306 NEW YORK STATE MUSEUM 



the usual one being that the pyroxenes and ilmenite increase in 

 amount and come to constitute from 15^ to 35^ of the rock, 

 which thus tends to approach gabbro in character. It is, how- 

 ever, much too feldspathic for normal gabbro and may well be 

 called anorthosite gabbro, since it represents a distinct inter- 

 mediate stage. With this mineralogic change there is always 

 found an accompanying change in texture, the rock becoming 

 always less coarsely crystalline, the big feldspars diminishing in 

 both number and size and the amount of granular mosaic increas- 

 ing. The rock also becomes more gneissoid. In this phase garnet 

 is sure to creep in, often in considerable quantity. It develops 

 in the main at the contacts between ilmenite and feldspar, the 

 one furnishing the iron, and the other the lime, alumina and 

 silica which enter into its formation. Garnets may form in this 

 way, either during the original cooling of the rock or owing to 

 subsequent metamorphism, and it is usually impossible to say, 

 in these anorthosites, whether they are to be referred to the one 

 or the other, or to both methods of formation. 



It is not meant to imply that changes of this sort occur only 

 as the edge of the mass is neared, but rather to point out that 

 they do uniformly occur under those conditions. But the same 

 rock is often produced well within the anorthosite mass by local 

 differentiation. It is in fact but a more extreme differentiation 

 of the same sort that gives rise to the local development of great 

 masses of titaniferous magnetite such as occur about Lake San- 

 ford in Essex county and have been described in detail by Kemp.^ 



This anorthosite gabbro forms the greater part of the boundary 

 of the Franklin county portion of the anorthosite mass. But on 

 the south a yet greater change takes place, and the anorthosite 

 gabbro passes over into a dark colored gabbro gneiss. This rock 

 has not yet been seen in fresh condition, and hence has»not re- 

 ceived the thorough investigation that its interest and importance 

 demand. The change is of the same sort as that involved in the 

 passage of anorthosite into anorthosite gabbro, but is more ex- 



^U. S. Geol. Sur. 19th An. Rep't, pt3, p.383-422. 



