156 
CONTRIBUTIONS TO THE GEOLOGY OF MAINE. 
[BULL. 165. 
III. Rhyolitc, Castle Mountain, Montana. Analysis by L. V. Pirrson. Weed and Pirrson, Geology 
of Castle Mountain mining district: Bull. United States Geological Survey No. 139, p. 120. 
IV. Rhyolite, Round Mountain, Colorado. Analysis by L. G. Eakins. Cross, Geology of Silver Cliff 
and Rosita Hills: Seventeenth Ann. Rept. United States Geological Survey, Part II, p. 324. 
V. Aporhyolite (Laminated orthofelsite) , South Mountain, Pennsylvania. Analysis by F. A. Genth. 
Second Pennsylvania Geological Survey, Vol. CCC, p. 265. Recognized as aporhyolite by F. 
Bascom, Bull. United States Geological Survey No. 136. 
VI. Aporhyolite, Fox Islands, Maine. Analysis by E. W. Magruder and W. A. Jones. G. O. Smith, 
Geology of Fox Islands, Maine. Presented as thesis at Johns Hopkins University; printed 
privately. 
The analysis (I) shows the Haystack rock to belong to the alkaline 
series of rhyolites, and a comparison with the other analyses presented 
(II-VI) shows it to differ in no important particular from typical 
modern and ancient rhyolites of other regions. As the microscopic 
examination indicated, the components present must be practically 
quartz and alkali feldspar. The percentages of the different minerals 
have been calculated from the analysis as follows: 
Mineral. 
Per cent. 
Quartz 
34.1 
26.2 
34.1 
2.7 
4. 
.4 
.5 
Orthoclase 
Albite 
Chlorite 
Kaolin 
Titanite 
Other minerals 
Total 
100 
WHITE TYPE. 
Near the base of the hill the rhyolite, while retaining the felsitic 
appearance of the main mass, becomes white in color and does not 
break into such keen-edged chips. The quartz aggregates filling the 
flattened cavities are easily seen in the hand specimen. The weathered 
rock is yellow-brown and shows numerous narrow, troughlike cavi- 
ties, at times becoming a quarter of an inch long and lined with small 
quartz crystals of secondary origin. 
The microscope reveals a structure of the same character as that found 
in the rock from the top of the hill. There are no phenocrysts of 
feldspar, and the pseudophenocrysts present are formed of aggregates 
of from 10 to 20 quartz grains, each of which retains its crystal out- 
line in some degree. As a rule, these aggregates have a large piece at 
the center, occupying perhaps one-half the space, and this piece is 
surrounded by smaller pieces, while still smaller fragments fill the 
irregular spaces next the cavity wall. The boundaries between the 
quartz grains are clear-cut lines, which often project into each other 
