GEOLOGY OF THE LAKE PLACID REGION 5Y 



thosite and that the transition from one to the other is a gradual 

 one. 



Anorthosites. The anorthosites may be considered to be the 

 characteristic rocks of theAdirondacks. In their typical cases they 

 consist of little else than blue labradorite and are then a most 

 beautiful rock. A little pyroxene, mostly augite, appears as a rule, 

 and hypersthene is frequent. The presence of the latter led Prof. 

 Emmons in the early work of the New York state survey to call 

 them hypersthene rock, or hypersthene, but this mineral is a 

 subordinate one, Labradorite is the great component and the 

 rocks might be fittingly described as ' labradorite rocks ', follow- 

 ing a custom prevalent in Norway, but in English the term is not 

 a good rock name. They are sometimes described as norite 

 meaning a rock composed of labradorite and hypersthene, but 

 experience has indicated the scarcity of hypersthene, and here 

 the word anorthosite is employed, which means a rock chiefly 

 composed of plagioclaise feldspar. The rocks resemble a coarse 

 granite, the individual crystals being sometimes very large. 



The typical anorthosites grade into varieties with more and 

 more dark silicates and some of the areas colored red on the map 

 have large and prominent amounts of them. These darker 

 pyroxenic and hornblendic rocks are not anorthosites, strictly 

 speaking but are gabbros and diorites; nevertheless the anor- 

 thosite is the prominent and characteristic variety and is here 

 used with that understanding. The summit of Mt Whiteface and 

 the southern portion of the mountain consists of a variety that 

 contains large amounts of hornblende and pyroxene, together with 

 milky white feldspar. It is so peculiar that we have been accus- 

 tomed in the field to refer to it as the Whiteface type of rock. It 

 is characteristic of this mountain ridge. Despite the peculiarities 

 of the rock, it belongs beyond question in the anorthosite series, 

 and is closely involved with typical anorthosites. The latter are 

 found all around the base of the mountain toward Wilmington 

 and on the trail from Wilmington village, to the summit, that 

 passes over Marble mountain, typical anorthosites appear till 

 the peak is nearly reached. 



