r64 NEW YORK STATE MUSEUM 



The eruptive core of the Adirondacks 



It has been shown in the preceding pages that the anorthosite- 

 tends to grade into a more gabbroic rock at its borders, becoming 

 at the same time much more gneissoid, and it is also the fact 

 that it fades out into the neighboring rocks through transition 

 phases, precisely as the syenite does. The Franklin county 

 anorthosite, the only large maeis with which the writer is per- 

 sonally familiar, passes at the south and southwest into a more 

 and more gneissoid rock, with constantly fewer and smaller 

 labradorite augen, which finally disappear, and the rock would 

 not be taken to have even a remote connection with anorthosite 

 by an observer coming on it from the opposite direction. Kemp 

 reports similar facts from Essex county. These border zones of 

 the anorthosite deserve more attention than they have yet 

 received. In the writer's district they have not yet been met 

 with under sufficiently favorable auspices to encourage detailed 

 work. All the exposures seen show the vo<tk much rotted and 

 altered. 



The gabbros (hyperites) illustrate the same thing. They fre- 

 quently show a more or less massive core, from which they pas» 

 into gneissoid gabbro diorites and amphibolites on all sides, and 

 these may become involved and interbanded with other rocks, so 

 that they can not be sharply separated in -mapping. 



Moreover they have a wide range in the Adirondacks, far 

 beyond that of the anorthosite, and thus negative any argument 

 seeking to separate the syenite from the anorthosite because of 

 its much wider range. The same reasoning would also separate 

 the gabbros, yet these are clearly later than the anorthosite. It 

 is therefore thought that the apparently less metamorphosed 

 character of the anorthosite is merely an apparent difference^ 

 likely a result of its peculiar chemical composition. 



But, if it be granted that these rocks, ranging from granite to» 

 gabbro, are all members of a great eruptive series of which the 

 anorthosite would seem to be the oldest member, interesting con- 

 sequences follow in regard to the distribution of the eruptives, 

 and in regard to the content of serie© 1 if these be abstracted. 



