PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 487 



Porphyryte. Tuff.] 



to determine whether the groundmass was originally holocrystalline. Two small 

 sections examined. 



Age. Cabotian. u. s. G. 



No. 679A. PORPHYRYTE. (Diabase.) 



Same locality as No. 678, which see. 

 Ref. Annual Report, x, page 76. 



Numerous brownish-gray porphyritic feldspars occur in a compact, very 

 fine-grained, almost black groundmass. 



.!//>. The section is closely similar to that of No. 679, but differs slightly in 

 having fewer phenocrysts and in the fact that the lath-shaped feldspars of the 

 groundmass are more numerous, more sharply defined and a little larger. 

 One section examined. 



Age. Cabotian. u. s. G. 



No. 680. TUFF. (Volcanic. ) 



Same locality as No. 678, which see. 

 Ref. Annual Report, x, pages 76, 77. 



^fey. There are two small hand samples with this number. One is a very fine- 

 grained, compact, reddish-brown rock with a few small red and gray porphyritic 

 feldspars; the rock is perhaps an andesyte. The other sample is of a brown color 

 and is thickly spotted with minute white areas. There are also some red feldspar 

 crystals. On closer examination the rock presents a fragmental aspect. The section 

 was evidently made from this second sample. 



Mic. The fragmental character of the section is evident at a glance. The 

 fragments are of very irregular outlines and they vary in size from mere shreds to 

 those about an eighth of an inch across. In color they vary from brownish semi- 

 transparent ones to those which are opaque and black. All the fragments have the 

 appearance of having been glass originally, but now are in part at least devitrified. 

 Some of them have a vesicular texture, the vesicles being filled with the materials 

 which make up the cement between the fragments. A few minute elongated feld- 

 spars are seen in some of the fragments, and in one these feldspars are arranged 

 somewhat as in a flow. One fragment contains a twin crystal, evidently of a glassy 

 feldspar cut nearly normal to an optic axis. A reddened feldspar is also among the 

 fragments. The cement of the rock is colorless to brownish or greenish, and is com- 

 posed largely of very fine-grained quartz, chlorite, a little epidote and probably/eWspar. 



It seems clear that this tuff is not an acid one, although its exact nature is not 

 evident. It seems quite likely, however, from its general nature and from the 

 character of the associated rocks that it is a rather basic andesyte tuff. 



One section examined. 



Age. Cabotian. 



