ADIRONDACK ROCKS IN GENERAL 403 



found only occasional small dikelike masses of granite or aplite and 

 numerous pegmatite dikes cutting the great syenite-granite series. After 

 all, my view of the intrusion of the syenite-granite series may not be very 

 different from that of Gushing, because I consider the tremendous body 

 of magma to have worked its way upward, slowly and irregularly, sending 

 out many offshoots which may have come to rest and congealed at slightly 

 different times. 



Still later than the syenite-granite series are the gabbro stocks and 

 dikes. In my experience these are mostly stocks, never more than a few 

 miles across, with elliptical ground-plan and practically vertical walls. 

 They usually exhibit finer grained amphibolitic border facies, while the 

 interior portions are usually medium to coarse grained with distinct 

 ophitic texture and more or less gneissoid. Such gabbro masses are com- 

 mon throughout the Adirondacks, except well within the anorthosite area. 

 The notable absence of Grenville strata, syenite-granite series, and gabbro 

 stocks from well within the area of the typical anorthosite all have impor- 

 tant bearings on the problem of the origin and structure of the Adiron- 

 dack anorthosite. 



Latest of all the intrusives is diabase in the form of small typical dikes 

 with nearly vertical walls. They are very common throughout the Adi- 

 rondacks, showing no marked tendency to be absent from the anorthosite 

 area. 



General Description of the Anorthosite 



extent of the anorthosite, area 



As far as definitely known, the anorthosite was the first of the great 

 intrusive bodies to break into the Grenville strata. As shown beyond in 

 this paper, the anorthosite is demonstrably older than the syenite-granite 

 series and intruded by the latter. With the exception of a few small out- 

 lying masses, this rock occupies a largely unbroken area of about 1,200 

 square miles of the central-eastern Adirondack region. 



MARCY TYPE OP THE ANORTHOSITE 



By far the most abundant facies of the rock I shall call Marcy anortho- 

 site because of its great exposures on Mount Marcy. The most typical 

 portion of the Marcy anorthosite is very coarse grained, light to dark 

 bluish gray, and consists very largely of basic plagioclase feldspar, mainly 

 labradorite. The dark bluish gray labradorite crystals usually vary in 

 length from a fraction of an inch to several inches, crystals about an inch 

 long being very common. Only occasionally do these labradorites exhibit 



