ar TACONIC QUESTION IN GEOLOGY. 231 
limestone, which were by Rogers included in the Matinal division of his classification. 
According to Prime, “the Trenton or fossiliferous limestone seems to occur only at 
a few points in the valley,” having been recognized by its fossils at one locality 
only. It is here dark-colored, earthy and uncrystalline, and associated with argillaceous 
beds which yield a hydraulic cement. These, which are supposed to belong to the same 
horizon, are found at several other places in the region, overlying the magnesian limestone. 
Prime also mentions one locality where forms referred to Ewomphalus and Maclurea are met 
with, indicating the horizon of the Chazy; while in another an undescribed Lingula 
occurs. Peculiar funnel-shaped markings, not very unlike the Scolithus of the underlying 
quartzites, have also been found in the magnesian limestone in one place, and have been 
referred to the genus Monocraterion ; which occurs in the Eophyton sandstone of Sweden. 
For farther notice of these organic forms see the author’s volume on Azote Rocks, p. 206, 
and also Prof. Prime’s Report D. 2. 
§ 35. The Primal division of the series under consideration is, in the north-east part 
of the great valley, in Pennsylvania, (where it rests uncomformably upon the Laurentian 
gneiss), a thin and irregular deposit, and, according to Rogers, is sometimes wanting ; in 
which case the Auroral limestone reposes directly upon the gneiss, as may be seen in 
Lehigh and Northampton counties. In the North-Valley Hill, in Chester county, and 
farther to the north-east, in Lehigh county, the Primal quartzite, often with Scolithus, is seen 
to rest, with a thickness of from twenty to fifty feet, directly upon the Laurentian gneiss. 
These basal beds in Chester county include some micaceous and schistose layers, and are 
followed by the Upper Primal slates and the Auroral limestones. The rock is sometimes 
granular, and often detrital, while at other times it is a hard granular or even flinty 
quartzite, Farther to the south-west, in Berks county, the Primal quartzite becomes more 
continuous and thicker, rising into high ridges. 
$ 36. The conditions above noticed show the deposition of these rocks over an uneven 
subsiding eneissic area, and a conformable overlapping of the Primal beds by the succeeding 
Auroral limestone. As described by the writer in 1876, “they were evidently deposited 
over a subsiding continent, with bold shores; so that while the Primal has in places a great 
thickness, it is elsewhere very thin, or entirely wanting beneath the Auroral, which rests 
directly upon the older crystalline rocks.” * The characters of the Primal are best 
seen farther to the west, where, in the broader part of the basin, it is brought up 
by undulations from beneath the Auroral, and appears as a complex group of considerable 
thickness ; with alternations of quartzites, argillites and crystalline schists, beds of iron- 
ores, and intercalated limestone-layers ; the latter constituting, as well described by Rogers, 
beds of passage into the overlying Auroral limestone. Rogers defined the group 
as a Primal sandstone, with slates above and below; but it is occasionally less simple, 
since what he called the Upper Primal slates may include interstratified sandstone-beds, 
sometimes of considerable thickness. Thus, in a section near Parkesburg, on the North- 
Valley Hill, described by him, a mass of 200 feet of yellow sandstone is found, with 300 
feet of slates above, and 350 feet more below, lying between this upper sandstone and 
the white Scolithus-sandstone beneath, which here measures fifty feet ; the section being 
as follows, numbered in descending order :— 


* Harper’s Annual Record for 1876, p. xevi. 
