100 The American Geologist. February, 1005 
leaving a white fibrous residue. It is assumed that the min- 
eral was augite because it exactly resembles the long blades 
of augite found in the peculiar variant of the Holyoke dia- 
base and called plumose diabase by the writer in a communi- 
cation to the Geological Society in 1903. The flat plate is 
formed by the large development of two opposite prism 
faces. The central suture is the common twinning plane. 
The transverse parting is the basal cleavage. All the mag- 
netite is concentrated around these blades and a few of the 
feldspars of the first generation in an unusually thick layer. 
The magnetite cleavage can be clearly seen on fragments 
treated with acid. The rock on this side allies itself to dia- 
base. On the other hand the dark flesh-colored spots which 
represent the ground have an aplitic aspect. A few ideo- 
morphic plagioclase crystals occur, rarely, in albite twins, 
mostly limpid but partly changed to a green mica in coarse 
scales. Anorthite and oligoclase could be determined optic- 
ally. A few long needles of a colorless hornblendic mineral 
appear. 
Surrounding these ideomorphic minerals is an abundant 
ground made up of an intimate mixture of quartz and feld- 
spar which imitates every form of the macroscopic pegma- 
tyte structure and avoids the worm-like, granopliyric or 
myrmekitic structure. The larger anhedral portions of 
quartz contain sparingly the rigid needles common in gran- 
itic quartz and send out into the feldspar long parallel equi- 
distant bars which intersect at angles of about 40 to 50 de- 
grees, and form a lace work of great beauty. In other feld- 
spars stouter quartz rods with many crystal faces produce 
exactly the aspect of common graphic granite. The whole 
is pale brown-dusted. Tliis ground has also a close resemb- 
lance to the ground in the plumose diabase mentioned above. 
It is there associated with palagonite which is wanting here. 
The micropegmatyte texture seems to depend on the unusual 
influence of water. 
Diabase Aphanxte. — A fresh, exceedingly fine-grained 
diabase, which would not be easily distinguished macroscop- 
ically from the finest varieties of our Triassic diabases ex- 
cept for the slightly brownish shade of color. Microscopic- 
ally one finds augite filling the crevices among the other 
