90 Watson — Dike of Diabase in the Potsdam Sandstone. 



The igneous rock was readily traceable for a distance of 100 

 feet or more in a nearly north-south direction, by large and small 

 bowlders on both sides of the wagon road, and by partially 

 decayed rock in place in the wagon road. A search in the 

 immediate vicinity of the exposure failed to reveal more of the 

 material. 



The rock is very dark, nearly black in color and is of dense 

 medium texture. A microscopic examination of a thin section 

 of it shows a typical olivine diabase in mineral composition 

 and texture. The principal constituents are augite, plagioclase, 

 feldspar, olivine, and magnetite. The lath-shaped feldspars 

 are enclosed by the crystalline augite in typical diabasic struc- 

 ture. Measurements of the extinction angle on 010 of the feld- 

 spar show it to be a basic plagioclase, a part of which corre- 

 sponds to labradorite and a part to bytownite, largely to the more 

 basic bytownite series. 



As to the age of the diabase, it can only be suggested that it 

 is Mesozoic, since it is in every respect entirely similar to the 

 diabase dikes found penetrating the Newark series of rocks in 

 Piedmont, Virginia, east of the Blue Ridge. It is entirely mas- 

 sive and unaltered, save from atmospheric agents, which have 

 resulted in the formation of a crust of clayey limonitic material 

 on most of the bowlders. 



Geological Department, Virginia Polytechnic Institute, 

 Blacksburg, Virginia, December, 1906. 



