22 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull 136. 
sandstone" of Rogers, and are represented by him as interbedded with 
slates and occurring in a series of narrow anticlines. They form the 
u Mountain Creek rock" of Frazer and Lesley, mapped by them as 
Azoic and represented both in sections and text as underlying the! 
"chlorite schists' 1 and "bedded orthofelsites." Messrs. Geiger and 
Keith, 1 in their earlier investigation of the southward extension of; 
this sandstone in the Blue Ridge, placed it above the limestone of the 
Cumberland Valley, and, upon structural grounds, determined its age 
as Upper Silurian (Medina). The true age of these sediments has 
recently been determined beyond question, through the discovery of 
fossils, by Mr. Walcott. 2 No fossils were found in the quartzose con- 
glomerates and quartzites of the Monterey district (No. 2 of Mr. Wal- 
cott's section), but the interbedded argillaceous shales, of which men 
tion has been made, are found to the west of the Monterey district pass- 
ing beneath quartzite and other shales in which were discovered the 
remains of Scolithus linearis, Olenellus, and Camerella minor. These 
discoveries undoubtedly refer the conformably underlying Monterej 
quartzite to a Lower Cambrian age. 
In a recent section made in the light of Mr. Walcott's discoveries anc 
of his own subsequent discovery of fossils in the Blue Ridge, Mr. Keith 
gives the Monterey sediments a similar position (No. 4), describing then 
as a "massive white sandstone with bluish black bands, feldspathic, anc 
in places conglomeratic." Mr. Walcott calls them "a coarse-grainec 
quartzite, sometimes conglomeratic." 
The sections (PL V) through the Monterey district show the relative 
distribution and position of the three type rocks. It will be observed 
that the sandstone lies in a gentle syncline, or almost flat, and wholly 
above the eruptives. These observations made by the writer in the 
Monterey district and elsewhere in the South Mountain, accord with the 
observations of Mr. Keith 4 in Maryland. Owing to the entire absence 
of exposure, save at the tunnel, the minor folding, which is undoubtedly 
present at many other points, is not indicated in the sections. 
Mr. Keith 3 finds that in Maryland, as a result of faulting, the igneous 
rocks occasionally overlie the sandstone. This is nowhere the case 
in the Monterey district. The facts that no inclusions of sandstone 
were found in the volcanic breccia, and that fragments of the acid erup- 
tive occur in the quartzose conglomerate, suggest the superficial char- 
acter of the latter. 
Further indications of the same nature will be discussed in consider- 
ing the comparative age of the sedimentary and igneous rocks. 
1 11. 11. Geiger and Arthur Keith, The structure of the Blue Ridge near Harpers Ferry : Bull. Geol. 
Soc. Am., Vol. II, p. 163, pis. 4 and 5. 
2 C. D. Walcott, Notes on Camhrian rocks of Pennsylvania and Maryland: Am. Jour. Sci., Vol 
XLIV, 1892, p. 481. 
3 A. Keith, Geologic structure of Blue Ridjre in Ma^land and Virginia: Am. Geologist, Vol. X, 18£ 
p. 305. 
4 Geiger and Keith, loc. cit., p. 155, pis. 4 and 5. 
6 Loc. cit.,p. 365. 
