WARREN. — ALKALI-GRANITES AND PORPHYRIES. 267 



porphyritic granite in the central portion of the westernmost dike, which has 

 been described in some detail in the text as being remarkable for the abundant 

 xenoliths of rhombenporphyry enclosed in the coarser grained part. The 

 original scale of this map has been reduced in reproducing it for this paper so 

 that 1 inch equals approximately 1000 ft. 



distinct points on one, or even both ends of the group. Minute dark 

 grains of pyroxene and specks of secondary minerals may be seen in 

 the feldspars, particularly about the margins. (See Figs. VIII and 

 IX, Plate 2). 



With a pocket lense the groundmass has a slightly oily lustre in 

 fresh specimens and is of a yellowish-black or to greenish-black color. 

 The grain is rendered somewhat indistinct by the alteration products 

 which are always present. Occasional grains of augite may be seen 

 together with tiny yellowish crystals of epidote, black magnetites 

 or ilmenites and sometimes patches of compact greenish black sec- 

 ondary material. Rarely a grain of pyrite occurs. Superficial 

 weathering causes the groundmass to rust (brown) and retreat leav- 

 ing the whitened and roughened feldspar in relief. 



Microscopic Characters. — The original minerals present in the type 

 analyzed are, soda-orthoclase,*° crytoperthite or microperthite and 

 augite with accessory quartz, apatite and magnetite or ilmenite. 

 With these are secondary green hornblende, epidote, biotite, titanite, 

 magnetite, calcite, sericite, pyrite and limonite. 



The habit of the feldspar phenocrysts at the close of the porphyritic 

 stage of grow^th seems to have been that of very acutely terminated 

 crystals with rounded contours, the crystal being frequently, in fact, 

 of almost lensiform outlines in cross-section. The tendency to form 

 composite groups is marked, the individual members being united by 

 quite irregular surfaces and often arranged in a slightly divergent 

 manner, and with separate terminations all pointing as a rule in the 

 same direction. The original outlines, doubtless those which give so 

 often the impression in the hand specimen of sharp boundaries, are 

 more or less obscured in thin section by the later growths of ground- 

 mass age. The central parts of the feldspar crystal^ are in part of 

 homogeneous structure, in part very finely striated and in part dis- 



40 The term anorthoclase will not be used here for these feldspars as 

 they appear to be monoclinic and the tei'm anorthoclase (used by Brogger for 

 closely similar feldspars in the rhombenpoiphyries of the Laurvikite area 

 Norway) implies triclinic symmetry. These feldspars may of course be 

 triclinic with a very small extiHction angle, in fact an apparent angle of ex- 

 tinction of 1° to 2° was observed on two or three basal cleavage fragments. 



