THE NEWER CORES AND SHORT DIKES. 485 



A plate of the feldspar was isolated and gave the optical characters of oitho- 

 clase and was wholly untwinned. 



The pocket lens shows grains and flat vein-like accnmnlations of a 

 deep-red, resin-like character. 



The microscope reveals anorthite, labradorite, augite, fayalite, mag- 

 netite, hematite, and, as inclusions, quartz, oi'thoclase, microcline, albite, 

 rutile, muscovite, and biotite, and fragments of granite and amphibolite. 

 The texture of the rock is entirely unlike any other occurrence among the 

 eruptives of the region. Tlie oi'iginal minerals enumerated appear as fresh 

 as in a modern lava, distantly, often very distantly, scattered in a linely 

 granular ground. 



Anorthite. — The porphyritic feldspars of the first consolidation are often 

 as much as 2°"" across. They seem to me to have shot out rapidly in thin 

 plates, which are often much bent and broken and the parts moved away 

 from each other, showing what is otherwise proved below, that they were 

 fomtied in the liquid magma and moved some distance in it before its entire 

 consolidation. They are of glassy clearness, and show not the faintest 

 trace of alteration. They are bounded by perfectly smooth crystal faces, 

 except where deep angular or rounded offshoots of the groundmass extend 

 into them. Inclusions of this groundmass are very abundant and variously 

 arranged — at times collected in the center, at times in concentric lines 

 marking old surfaces of the crystal, which often differ from the final form; 

 at times quite regularly airanged in I , T, and L shaped masses, conforming 

 to the cleavage planes with great regularity, the masses being of quite uni- 

 form size. The crystals show lines of growth of great delicacy, which are 

 curiously disturbed as they bend in to surround rounded projections of the 

 groundmass which penetrate the crystal. Some of the broadest plates lie 

 so that they show no twinning, indicating that they are broad by the large 

 development ooPo). The angle of extinction is —36° on either side the 

 twinning plane, indicating anorthite. 



Labradorite. — The feldspars of later consolidation, from J°"° to the 

 smallest dimensions, extinguish with an angle of 12^° on either side the 

 twinning plane, and so are very probably labradorite. They affect the lath 

 shape more than the anorthite does. The larger are, however, broad, 

 with square ends; the smaller, nan-ow rods; and they are rarely in contact, 

 so that the ophitic structure characteristic of the common diabase is wanting. 



