TACONIC QUESTION IN GEOLOGY. 233 
Azoic rocks were not clearly defined, and that, in his opinion, it was often difficult, if not 
impossible, to distinguish between the upper portions of the Hypozoic and certain parts of 
the Azoic series. It is clear from his descriptions, and from my own examinations in the 
region, that portions of Huronian, and of Montalban, were by him included in the Hypozoic ; 
and other portions of the same, or of older rocks, in the Azoic, or even in the Upper 
Primal slates. Both these, and the Primal quartzite itself, were by Rogers supposed to 
have been changed into feldspathic rocks; and he has described as altered Upper Primal, a 
great group of such rocks seen in the South Mountain to the south of the Susquehanna, 
which we shall proceed to notice. 
§ 40. Leaving the Mesozoic red sandstones at Gettysburg, and passing westward over 
the South Mountain, by Caledonia Spring to Chambersburg, we meet first with a belt, more 
than two miles wide, of crystalline rocks, regarded by Rogers as in part Upper and in part 
Lower Primal slates ; the latter represented by talcose, chloritic and epidotie schists, with 
diorites, and the former by what were called by Rogers, “jaspery rocks,” and “reddish 
jaspery slates.” These, which I first saw with Dr. Persifor Frazer in 1875, were found to 
consist of petrosilex or compact orthofelsite, often becoming porphyritic from the presence 
of crystals of feldspar or of quartz. I then compared them with the similar rocks found 
along the coasts of Massachusetts and New Brunswick, and on Lake Superior, all of which 
J at that time included in the lower part of the Huronian, bnt have since been led to 
regard as an independent series, identical with the Arvonian of Hicks ; which, in Wales, 
appears to be interposed unconformably between the Laurentian (Dimetian) below, and 
the Huronian (Pebidian) above. 
§ 41. To this series also belongs a great thickness of petrosilex-rocks, often porphyritic, 
and associated with small portions of soft unctuous micaceous schists, occuring in 
central Wisconsin ; where they overlie conformably a great mass of vitreous quartzites, 
which, from the intercalation of similar micaceous layers, apparently belong to the same 
series with the petrosilex. These rocks, originally described by Percival as altered Pots- 
dam sandstone, were by James Hall, in 1862, referred to the Huronian, with which they 
are also classed by Irving, who has lately described them.* I have recently examined 
these rocks, in situ, as seen on the Baraboo river in Wisconsin, and have found them indis- 
tinguishable from the petrosilex-beds of Pennsylvania, and of our Atlantic coast, and 
from the typical Arvonian of Wales. 
§ 42. These petrosilicious strata, presenting many varieties in colour and in texture, have 
a great thickness in the South Mountain, west of Gettysburg, where they generally dip 
south-eastward at high angles. With them are seen in some parts, apparently interstra- 
tified, thin bands of argillite, with chloritic and epidotic rocks, such as I have found with 
the similar petrosilicious rocks on Passamaquoddy Bay, on the Atlantic coast. This crys- 
talline series is, to the westward, unconformably overlaid by a belt about a mile and a half 
wide, of sandstone, with conglomerates, generally with a north-western dip ; constituting 
what is known as Green Ridge. This is followed by a repetition of the petrosilicious 
rocks, again with high south-east dips, and by a great mass of chloritic and epidotic strata, 
overlaid to the westward, as before, by a considerable thickness of Primal sandstone, 

*See Geology of Wisconsin, 1877, vol. ii. pp. 501-521; also Hunt, Azoic Rocks, p. 232. 
Sec. IV., 1883. 30. 
