PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 541 



Gabbro. Syenyte.] 



" No. 780 is a form of the rock in the same bluff about twenty feet below the top; evidently a part of the 

 igneous rock, but affected by proximity to the underlying roek so as to appear like a dioryte. 



" There is a striking contrast in the appearance, form and color of the rocks Nos. 777 and 779." 

 Ref. Annual Report, x, pages 100, 101; Bulletin ii, pages 91, 92, 94, plate IV, figure 1. 



Meg. A fine-grained, dark-gray crystalline rock, which contains a number of 

 porphyritic plagioclases. These porphyritic crystals, on account of a color similar 

 to that of the mass of the rock, are not very evident in the hand sample. 



Mic. M. E. Wadsworth's description of this rock is as follows:* 



" Is grayish brown, porphyritic, resembling that found in Gloucester, Mass., and 

 belongs to the gabbros. The section is composed of large plagioclase crystals, lying 

 in a crystalline granular groundmass of ilniUaije, aiiyite, biotite, HraUte, magnetite, 

 ]>Itif/ioclse, orthoclase, and some <ji/rtz. All are in more or less rounded grains, and 

 the general structure of the groundmass is that of one which has been formed by the 

 recrystallization of its materials, /. e., the augite and diallage are the remnants of 

 larger crystals, while much of the feldspar and all of the quartz, biotite and uralite 

 are the results of alteration. The biotite and uralite are formed from the pyroxene. 

 The larger plagioclase crystals are filled with elongated dark needle-like shapes which 

 are arranged parallel to the twinning, clinopinacoid, and other crystal planes. These 

 needles polarize with a bright, yellowish-brown color, like thin biotite plates. On 

 further examination some sections are found cut obliquely to these needles, when 

 they are found to be elongated, oval, yellowish-brown plates of biotite. These plates 

 are evidently an alteration product of the feldspar itself. 



"Plate IV, figure 1, shows the granular structure of this rook with the brown 

 biotite on the right and greenish hornblende at the bottom of the figure. The section 

 in places shows the characters of a dioryte or hornblende schist." 



The texture of the groundmass of this rock is very similar to that of No. 698; 

 but in No. 777 the grain is a little finer and the plagioclase has more of a tendency 

 to a lath-shaped development, giving the rock a diabase-like appearance, and thus 

 the rock might be called a diabase porphyryte. 



The porphyritic feldspars have equal extinction angles in sections cut normal 

 to 010 as high as 33, and a cleavage flake parallel to 010 gave an extinction of 20. 

 Both indicate labradoi-ifc with approximately the composition Ab, An,. The 

 feldspar of the groundmass was not determined carefully, but it appears to be 

 labradorite. 



Two sections examined. 



Age. Cabotian. u. s. G. 



No. 778. SYENYTE(?) (wiili 



Same locality as No. 777. 



Ref. Annual Report, x, pages 100, 101; Bulletin ii, page 120. 



"Bulletin ii, pp. 91, 2. 



