BASIC VOLCANICS OF HEMLOCK FOEMATI055". HI 



color. In places the varioles are in juxtaposition. However, they do not 

 coalesce, but each is separate and distinct (fig. J5, PI. X). 



The rock when examined under the microscope is seen to be extremely 

 altered. The only original minerals present are feldspar, apatite, and pos- 

 sibly some magnetite. 



The groundmass consists of a finely crystalline secondary aggregate 

 of flakes of chlorite, associated with minute limpid grains, some of which 

 are probably quartz and others feldspar. Scattered tlu-ough this aggregate 

 are grains of epidote, calcite, a few crystals of original apatite, and mag- 

 netite, and numerous dark reddish brown and black ferruginous specks. 



The varioles are readily distinguishable from the matrix. From this, 

 as well as from each other, they are invariably separated by a crack, along 

 which reddish-brown ferruginous matter has been infiltrated. The varioles 

 are in general much finer grained than the groundmass, and at times 

 exhibit phenocrysts of feldspar. The composition of the varioles is the 

 same as that of the groundmass, except that apatite is more common in 

 them, and that in addition to the minerals mentioned as occurring in the 

 groundmass a small quantity of original feldspar may be recognized, both 

 as phenocrysts and as part of the groundmass of the varioles. Where 

 these feldspars occur, they are to a great extent replaced by a mass of epi- 

 dote, chlorite, sericite, quartz, and feldspar. The phenocrysts are found 

 near the center of the varioles, and the occasional light-colored centers 

 which were observed macroscopically are due to the presence of these 

 altered feldspar phenocrysts. The more frequent dark centers are due to 

 an accimiulation of the dark ferruginous specks in varioles in which the 

 phenocrysts are wanting. 



No textures could be determined from the remnants of the original 

 minerals. In one variole aggregates of secondary epidote grains and fer- 

 ruginous specks lie in such a position as to produce a distinct radial 

 arrangement. With advancing alteration, spherulites in acid rocks are 

 frequently found to have between their radial fibers secondary deposits of 

 epidote and ferruginous matter, wliich mark very clearly their radial 

 arrangement. The similar radial arrangement in these varioles of epidote 

 and ferruginous matter seems to point to the- varioles having possessed the 

 spherulitic character, though it is now impossible to determine the nature 

 of the fibers forming the spherulites. 



