78 PROCEEDINGS OF THE ACADEMY OF [1890. 



A third common variety of the gneiss of this group is a micaceous quartzose rock, 

 generally of a light gray color. Some beds of this species contain such a predom- 

 inance of the crystalline quartz, in minute granular divisions, and such a subordi- 

 nate amount of minutely disseminated mica, as to have a character of ordinary gray 

 whetstone ; but this species of the gneiss is much more abundant in the middle 

 belt than in the southern one. A much coarser kind of gray micaceous gneiss, con- 

 sisting of a predominance of rather large flakes of mica, with a subordinate quan- 

 tity of feldspar and quartz, occurs interstratihed with all these other species, as a 

 very usual transition variety between the standard gray gneiss and the highly mica- 

 ceous kinds verging toward mica-slate. 



It is very usual to find the typical gneiss, of a three-fold constitution, alternating 

 with the hornblende species, and both of these alternating with the quartzose mica- 

 ceous variety. As a general fact, not without exceptions, however, the more mica- 

 ceous the rock is, the greater is the abundance in insulated crystals of common gar- 

 net. 



Interposed among the above varieties of true gneiss are beds, more or less thick, 

 of kinds so abounding in mica as to be entitled to the designation of true mica- 

 slate. This rock prevails in two or three outcrops, both above and below Colum- 

 bia Bridge, and it may be stated generally, that the further north we advance across 

 the southern division of the gneiss, the larger is the proportion of the more mica- 

 ceous varieties of the ordinary gneiss, and the greater the frequency of these bands 

 of mica-slate. 



An interesting variety of the ordinary or more feldspathic kind, is one containing 

 large, more or less insulated segregations of crystalline feldspar, the longer axes of 

 whose crystals lie generally parallel with the lamination of the rock. This variety 

 may be designated as a porphyritic gneiss, having that feature of an excess of crys- 

 talline feldspar which is accepted by geologists as a distinctive character of the por- 

 phyritic rocks and being, moreover, essentially similar to those well-known and 

 beautiful granites which geologists agree to call Porphyritic. 



A band of this porphyritic gneiss occurs at the Falls of Schuylkill, just below 

 the quarries, and ranges toward Nicetown in one direction, and toward the 

 tollgate at the Lancaster Road, five miles west of Philadelphia, in the other. 

 Another outcrop of the same feldspathic variety of the gneiss may be seen 

 crossing the West Chester Plank Readjust east of Darby Creek. 



The Second or Middle Belt. The middle zone of the gneiss of Southern 

 Pennsylvania, as it crosses the Schuylkill, consists of an alternation of four princi- 

 pal varieties. Perhaps the most abundant of these is a very micaceous species of 

 the ternary or mica, quartz and feldspar rock, holding garnets in greater or less pro- 

 fusion. A very common feature in this rock is a wavy or minutely undulated 

 lamination, arising apparently from a contorted or wavy structure in the coarsely 

 crystallized mica, its predominant mineral. This would seem to proceed from the 

 interference of the innumerable planes of cleavage, or — what is the same thing — of 

 crystalline lamination, with the original planes of the deposition of strata. The 

 twisted forms of the flakes of mica are frequently seen to be due to the displacing 

 effect of grains, or crystalline bunches of included quartz. It would seem as if 

 these minerals had crystallized or segregated, from their parent sedimentary 

 materials, under a conflict of forces, the newly awakened crystalline energies 

 being not always parallel to the original bedding of the deposit, but more fre- 

 quently oblique to it. 



Perhaps the most common subdivision of the gneiss of this middle tract is a 

 variety consisting almost exclusively of the above described wavy mica. This 

 rock graduates into the more micaceous sorts of gneiss, by containing a less or 

 greater mixture of finely granulated crystalline quartz or feldspar or hornblende. 

 The southern half of the gneissic zone before us is characterized on the Schuyl- 

 kill and Wissahickon, by containing an alternation of the above two varieties of 

 micaceous gneiss or mica-slate, with beds of hornblende gneiss, the last-named 

 rock being, from its thin lamination, sometimes entitled to the name of hornblende 

 slate. The northern half of the zone consists largely of a fourth variety of the 



