864 The American (jeoliKjixi. De<eini>er, ism 
knobs in a synclinal axis lU'xt the xallcv linicstont'; the massive 
sandstone follows the syneline of the main ridge; and between the 
two sandstones lies a slope or valley of shales. Southward fi-om 
the Potomac the two sandstone types l)ecome very much alike; 
northward the}- diAerge and their ditferences are accented. 
(Jn structural grounds alone the two sandstones should be equal 
to each other in age as the}' occupy parallel synclin(^s and the 
same shale passes under both. 
On this hypothesis, the massive bed would represent shore accu- 
mulations and the fine sandstone the otf-shore deposit. Even then 
the narrow limits of the change render it ver}- alnnipt. 
Organic evidence, however, does not liear this out. As has 
been stated, fossils were very numerous in the fine sandstone 
and a few were found in the sandy shales. In the massive sand- 
stone and the black slaty shale beneath it none were discovered. 
This difference in organic contents is as marked and persistent as 
the textural difference between the beds, and could hardly have 
been possible in synchronous deposits a mile or less apart. 
The age of the massive bed of sandstone must be determined by 
structure in default of fossils. Over the fossiliferous beds lay 
the valley limestone, as determined by the sequence at York, Pa., 
and at Balcony Falls, Va. If, then, the limestone lay directly 
over the massive sandstones, the two sandstones must Ije the same 
in ao-e ; if no limestone rested on the sandstone the latter would 
presumably be older than the sandy shale. In the latter case 
sandy shales might be expected to occur above the sandstone at 
some point. Search was made for such places and several were 
found, two of them being indubitable. One was immediately 
northwest of Monterey, Pa., on the Blue Ridge, near the Maryland 
line- the other was on the Blue llidge, six miles northeast of Front 
Royal, in Virginia. In each of these cases a syneline of the 
massive quartzite was overlain by sandy shales for a few miles. 
A similar sequence extends along the east side of Catoctin mount- 
ain for about ten miles and appears to lie complete west of Fred- 
erick. There all the sandstones and shales of the mountain dip 
east under the Frederick limestone. The resemblance between 
this section and the York section is striking, and each contains 
the same varieties of massive and slaty limestones and limestone 
conglomerates. The York limestone is largely Cambrian and the 
resemblances of structure and composition indicate that the Fred- 
