376 E. T. WHERRY PRECAMBRIAN OF PENNSYLVANIA 



through the northern part of the city of Philadelphia to the Delaware 

 River at Trenton, Xew Jersey, where it is likewise covered by later sedi- 

 ments; and the Highland belt, which emerges from beneath the sedimen- 

 tary cover 40 miles east of Harrisbnrg, and at Easton crosses the Dela- 

 ware River into Xew Jerse}^, where it broadens to form the Highlands. 

 The last belt, to which attention is especially directed in this paper, is 

 about 60 miles long and attains a maximum width of 20 miles in this 

 State. 



Henry D. Rogers, first State Geologist of Pennsylvania, regarded the 

 Precambrian rocks of all of these belts as dominantly sedimentary in 

 origin, stating:^ 



"The strata upon wliich the surface-wash and soils of Pennsylvania repose 

 belong to the three oldest classes of the sedimentary rocks — namely, to the 

 gneissic, the Paleozoic, and the earliest Mesozoic. ... No large masses of 

 igneous or volcanic rocks of any description appear within the borders of the 

 State." . . . 



J. Peter Lesley, the second State Geologist, evidently held the same 

 view, for in his chapter, "Are the Archean rocks sedimentary ?" ^ he dis- 

 cussed the banded structure of the gneisses and their intimate association 

 with cr3^stalline limestone and supposedly sedimentary serpentine and 

 iron ore, and presumably concluded that the gneisses are in general of 

 sedimentary origin, without so stating directly. 



In late years, however, the tendency has been to interpret the gneisses 

 of the Highlands of New Jerse}', and correspondingly of the adjacent 

 Pennsylvania belt, as of dominantly igneous origin, as illustrated by 

 three folios.* The presence of Precambrian sediments, represented by 

 the Franklin limestone and certain of the gneisses, was also recognized in 

 these folios, as well as in a separate paper by Professor Bayley.*" Some 

 of the occurrences in the ISTew Jersey Highlands, referred to in that paper, 

 are evidently very similar to those in the Pennsylvania area here de- 

 scribed. 



More recently it has been pointed out by Dr. C. N". Fenner^ that the 

 banding of some of the supposedly primary gneisses is of such a char- 

 acter "that their origin must be looked for in a process involving the in- 

 jection of a thinly fluid granitic magma between the layers of an original 

 rock of laminated structure.*^ 



- Geology of Pennsylvania, vol. 1, 1858, p. 59. 



3 Summary Final Keport, Second Pennsylvania Geological Survey, vol. i, 1892, pp. 95- 

 117. 



% W. S. Bayley : Kept. 12tli Int. Geol. Congress, 1913, p. 334. 



* A. C. Spencer, W. S. Bayley, and others : U. S. Geological Survey, Passaic, Franklin 

 Furnace, and Raritan Folios, Nos. 157, 161, and 191, 1908-1913. 



5 Journal Geology, vol. 22, 1914, pp. 594-612, 694-702. 



