TACONIC QUESTION IN GEOLOGY. 235 
in the Primal slates are apparently feldspathic in composition, since they are changed by 
sub-aérial decay into clays resembling kaolin. 
§ 45. The continuous belt of Primal and Auroral rocks stretching along the south-east 
base of the North or Kittatinny Mountain, is bounded on the south, in its extension between 
the Delaware and Schuylkill rivers, by the so-called South Mountain. Beyond the Schuylkill, 
at Reading, this Laurentian range is, so far as known, represented only by one small mass, a 
little west of the town. Its disappearance at the Schuylkill, to rise again south of the 
mesozoic belt, in the northern part of Chester county, permits a great extension of breadth 
ofthe Primal and Auroral rocks to the southward, in the counties of Chester, Lancaster, 
and York; where they appear, both to the north and the south, from beneath the broad 
and somewhat irregular belt of mesozoic sandstone which, from the Delaware to the 
Susquehanna, crosses the state in an east and west direction. From the Susquehanna to the 
line of Maryland, however, the trend of this belt is to the south-west. The Primal and 
Auroral strata, along the south and east of the mesozoic, occupy the limestone-valleys of 
Lancaster and York counties, with which the narrow limestone-valley of Chester county, 
lying to the eastward, is, as Frazer has shown, continuous. 
§ 46. The South Mountain, which, as we have seen, is effaced between the Schuylkill 
and the Susquehanna, re-appears to the south-west of this river in the broad ridge of 
crystalline rocks, already described in § 40, as found in Adams county, between the contin- 
uous limestone-valley on the north-west, and the mesozoic on the south-east. In this ridge 
of Huronian and Arvonian rotks, the Laurentian has not yet been recognized. It, however, 
as already remarked, appears in Chester county, between the mesozoic and the Chester 
limestone-valley. In addition to this, I pointed out in 1876 the existence of a subordinate 
Laurentian axis south of the limestone-valley just named, crossing the Schuylkill in Buck 
Ridge, near Conshohocken. * This ridge bears upon both flanks the Montalban gneisses 
and mica-schists; while between these and the Laurentian, on the south side of the axis, 
there is seen on the river, an intermediate mass of hornblendic and chloritic schists, with 
serpentine, enstatite and steatite ; which may be an intervening outcrop of Huronian. 
§ 47. Returning now to the Primal and Auroral rocks, the distribution of which has 
been defined, we remark that it is chiefly along the border of the mesozoic belt that the 
Primal schists, with their accompanying crystalline iron ores, already noticed, (§ 26), are 
best exposed. Examples of these ores are seen at Boyerstown, and near Reading, at Wheat- 
land, Cornwall, and Dillsburg, on the north side; and at the Warwick and Jones mines, on 
the south side, of the mesozoic sandstone. Rogers, in his third annual report on the geo- 
logy of Pennsylvania, in 1839, referred these iron-ores to the mesozoic or “ middle secondary 
red sandstone ” series ; giving, as examples, besides the mines just mentioned on the south 
side of this belt, the Cornwall mine on the north side. In his final report, in 1858, how- 
ever, he referred these crystalline iron-ores and their enclosing schists to the Upper Primal 
slates. He regarded the iron as an original constituent of the sediments, but supposed it to 
have been re-arranged “by some agency connected with the metamorphism of the strata,” 
Lesley, in 1859, in his /ron-Manufacturers Guide, described these same ores under the head 
of “ Primary,” with those of the gneisses and pre-paleozoic crystalline rocks ; at the same 
time referring with approval to those who regard these ores “as of middle secondary, and 


* Azoic Rocks, page 200. 
