bascom.] THE SEDIMENTARIES. 21 
The slates of the region thus prove to be both sedimentary and 
igneous. The former are argillaceous. The latter are either acid or 
basic, and are far more abundant than the former. 
SEDIMENTARY ROCKS. 
Area! distribution. — The first-mentioned of the rock types occupies 
the high altitudes only, Green Ridge, Monterey Peak, Pine Mountain, 
and Jacks Mountain being largely formed of it, while it caps Haycock 
Mountain and the foothill to the east of Jacks Mountain. 
Structural features. — The strike of the sediments is uniformly north- 
west and southeast, with a mean dip of about 45 degrees southeast. 
There are some exceptious to this southeasterly dip, notably at Monterey 
Peak, where the dip is northwest, forming, with the Green Ridge sand- 
stone, a gentle syncline. (See PI. Y.) 
In the Jacks Mountain quartzite the bedding is exceedingly obscure 
and the cleavage very marked. There is an opportunity for error in 
determining the dip by the confusion of the two. 
At the northeast end of the tunnel which is cut through a spur of 
Jacks Mountain, on the Gettysburg Railroad, stratification is dis- 
tinctly visible, and shows that minor crumpling of the sediments as 
well as folding on a large scale resulted from their uplift. The rock 
is here folded in a series of small anticlines and synclines. (See PI. IY.) 
Thickness. — Professor Rogers estimated the thickness of the sand- 
stone at 1,000 feet. Professor Lesley, on the other hand, considers it 
"immensely thick,'' and states that Frazer's " section 11" shows 32,000 
feet of quartzite and 61,000 feet of schistose conglomerate. 1 
Walcott 2 and Keith, 3 in their examination of a section of the same 
quartzite, sandstone, and conglomerate, displayed to the west of the 
Monterey district, agree essentially with Professor Rogers's estimate 
(1,000-1,200 feet). The writer did not find sufficient data within the 
Monterey district for a reasonably correct estimate of the thickness of 
the quartzite. There are undoubtedly a very great number of minor 
crumplings and foldings resulting in compressed synclines, anticlines, 
and thrust planes in Jacks Mountain. Since there are no means of 
determining the number of these folds, no estimate can be made of the 
thickness of the formation. If the minor folds are ignored, the dip and 
horizontal extent of the sediments indicate a thickness which is enor- 
mous, an indication unsustained by sections only 2 miles to the west. 
Probably the estimate made by Mr. Walcott (1,200 feet) in that more 
favorable locality will be approximately correct for the Jacks Mountain 
quartzite. 
Age and superposition. — These siliceous elastics are the " Primal white 
1 Summary, Final Report Geol. Pa., Vol. 1, 1892, pp. 145-146. 
2 Walcott, Notes on the Cambrian rocks of Pennsylvania and Maryland from the Susquehanna to the 
Potomac: Am. Jour. Sci., Vol. XLIV, Dec, 1892, p. 481. 
3 Keith, The geologic structure of the Blue Ridge in Maryland and Virginia: Am. Geologist, Dec, 
1892, Vol. X, No. 6, p. 365. 
