CHAPTER III. 
PETROGRAPHICAL DESCRIPTION OF THE CAMBRIAN ROCKS. 
MACROSCOPICAL DESCRIPTION. 
The sedimentary formation exhibits two marked phases, the con- 
glomeratic and the qnartzose. Its lowest member is conglomeratic. 
These conglomerates are frequently slaty, through the development 
of more or less sericite. They contain quartz pebbles from an inch to 
an inch and a half in length, and sometimes show included fragments 
of quartz-porphyry and of a green slate. 
From a conglomeratic character the sediments pass through a coarse 
sandstone into a compact quartzite exhibiting under the microscope the 
characteristics of a r eery stalli zed clastic. 
The qnartzose sandstone is exposed in great masses on the flanks and 
summit of Monterey Peak, on Haycock Summit, at the Gladhills Switch 
on the Gettysburg Railroad and at many points along the railroad 
as it skirts the east side of Jacks Mountain, and in massive pinnacles 
along the crest of that mountain. That the sandstone has been greatly 
fissured and broken is indicated by the conspicuous and frequent quartz 
veining, but metamorphism, save in a few cases, has been limited to 
the formation of a vitreous quartzite by the deposition of a siliceous 
cement. The original stratification planes are usually preserved, and 
cross bedding is frequently conspicuous. A secondary cleavage is also 
very marked, so much so as, in the absence of well-defined stratifica- 
tion planes, to be mistaken for the bedding. 
MICROSCOPICAL DESCRIPTION. 
QUARTZITE. 
Four specimens of typical quartzite were studied in the thin section. 
They were obtained from widely separated localities, and range in color 
from gray to a light green. Two of them illustrate the enlargement of 
quartz fragments and the genesis of a quartzite as perfectly as do any 
of the Lake Superior sandstones. 1 (PI. XV, a.) 
1 Qnartz enlargements were first described by Tornebohm, subsequently by Sorby, Young, Irving 
and Van Hise, Bonney, Phillips, and Iddings: 
A. E. Tornebohm, Ein Eeitrag zur Frago der Quartzbildung : Geol. Foren. Stockh. 1876, Vol. Ill, 
p. 35. Kef. Neues Jahrbueb fur Mineral., 1877, p. 210. 
II. Clifton Sorby: Anniversary Address, Quart. Jour. Geol. Soe. London, Vol. XXXVI, 1880, p. 62. 
A. A. Young: Am. Jour. Sci., A r ol. XXII, July, 1881. 
It. I). Irving: Am. Jour. Sci., Vol. XXV, June, 1883. 
E. D. Irving and C. R. Van Hise: Bull. U. S. Geol. Survey No. 8, 1884. 
T. G. Bonney and J. A. Phillips: Quart. Jour. Geol. Soc. London, Vol. XXXIX, 1883, p. 19. 
E. U. Irving and C. E. Van Hise: The Penokee iron-bearing series of Michigan and Wisconsin; 
Tenth Ann. Kept. U. S. Geol. Survey (1888-89), p. 375, I']. XXVII, fig. 2. 
J. P. Iddings: Mon. TJ. S, Geol. Survey, Vol. XX, 1892, pp. 346-347. 
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