124 
CONTRIBUTIONS TO THE GEOLOGY OF MAINE. 
[bull. 165. 
Analysis. — The following analysis of tuff of the normal type from £ 
Castle Hill was made by Dr. W. F. Hillebrand, of the United States 
Geological Survey: 
Analysis of tuff of the normal type from Castle Hill. 
Constituent. 
Per cent. 
Constituent. 
Per cent. 
Si0 2 
31.42 
11.57 
2.37 
7.48 
5.32 
16.71 
2.26 
.74 
.76 
4.17 
2.30 
None. 
Trace. 
VoO, 
.06 
Trace. 
.38 
.64 
None. 
Trace. 
.46 
13. 13 
? 
? 
.16 
A1 2 3 
NiO 
Fe 2 3 
MnO 
FeO 
BaO 
MgO 
SrO 
Li 2 
CaO 
Na 2 
P 2 5 
K 2 
C0 2 
H 2 O-105° 
CI 
H 2 O+105° 
Fe 
Ti0 2 
FeS 
Zr0 2 
99.93 
O 2 3 ._-- 
Discussion of analysis. — In regard to the barium present in the tuff, 
Dr. Hillebrand says: u The barium exists as silicate, since the sulphur 
is all pyritic, and it does not appear as carbonate." 
Silicified tuff of Castle Hill.— The specimens of this tuff from the 
ledge east of the mouth of Welts Brook are black, exceedingly dense, 
hornstonelike rocks, with a fracture approaching the conchoidal, and 
have the appearance of metamorphosed siliceous breccias. The black 
groundmass is mottled with light greenish-gray areas, which are proved 
to be glasss by microscopic examination. Numerous cubes of pyrite 
with striated faces form a prominent macroscopic feature. Areas a 
square inch or more in extent, composed largely of small grains of 
jasperlike material, appear on the surface. Quartz-filled cracks inter- 
sect the specimen, and weathering does not proceed rapidly on exposed 
surfaces. 
Microscopically the rock is practically a mass of glassy fragments 
thoroughly cemented together with silica. The larger and many of the 
smaller lapilli are greenish yellow in the section and are sometimes 2 
millimeters in diameter. They are divided into polygonal areas by dark 
lines, which appear to be incipient cracks filled with opaque material. 
The glassy fragments are broken and separated into distinct pieces, or 
split part way across, leaving wide cracks, now filled with siliceous 
cement. Areas are found composed of a mass of lapilli with cusps 
and concave spaces forming the outer border and having vesicular 
