34 NEW YORK STATE MUSEUM 



resistant than the great mass of surrounding practically undis- 

 turbed Grenville strata. This satisfactorily explains not only why 

 syenite-granite masses are scarcer within the anorthosite area than 

 in the Adirondack region in general, but also why syenite-granite 

 is almost, or entirely, absent from the southwestern half of the 

 anorthosite area. May not there have been a wide magmatic feed- 

 ing channel extending northwest by southeast under the main 

 body of the southwestern half of the anorthosite? On this view, 

 the thickest portion of the laccolith developed directly over the 

 wide feeding channel which extended far down, with the result 

 that this portion of the anorthosite intrusive body was very resistant 

 to intrusion by the syenite-granite magma. The northeastern por- 

 tion of the anorthosite, because notably thinner, was penetrated by 

 considerable masses of the syenite-granite magma, as, for example, 

 in the Lake Placid and Ausable quadrangles. Here again we have 

 a simple explanation of the field 'facts. 



Strongly supporting the above conception* is the evidence from 

 the distribution of the stocks of later gabbro. All the recent 

 workers in Adirondack geology recognize this gabbro as distinctly 

 younger than the syenite-granite series. It usually occurs in the 

 form of stocks or pipelike bodies rarely more than a few miles 

 across. Such stocks are common and widespread throughout the 

 Adirondack region, except the anorthosite area. Like the syenite- 

 granite, this gabbro is singularly absent from the southwestern half 

 of the anorthosite area. In the Schroon Lake and Elizabethtown 

 quadrangles a number of such gabbro stocks each from 2 to 4 miles 

 long lie right along the border of the anorthosite but none well 

 within it. In the northeastern half of the anorthosite area gabbro 

 stocks occur in moderate size and number. It is, then, very clear 

 that this later gabbro shows the same sort of distribution with 

 reference to the anorthosite as does the syenite-granite, and it is 

 believed that the same explanation (see above) applies to both. 

 Evidently the gabbro intrusions, too, were unable to penetrate the 

 thick, very resistant southwestern half of the anorthosite 

 laccolith. 



Origin of the anorthosite by differentiation in a laccolith of 

 gabbroid magma. Laccolithic structure df the anorthosite. After 

 considering a number of the better known anorthosite bodies of the 

 world, Daly 1 concludes that all of them, including the Adirondack 

 mass, are to be regarded as laccoliths. 



2 Igneous Rocks and Their Origin, p. 328-35. 1914. 



