;trtti> 



PENNSYLVANIA, 



471 



pebbles of Ilurouian rocks. (Second Gcolojrical Survey of Pennsylvania, 



D, p. QQ.) 



In 187G, he says that the middle zone of gneiss of Rogers is "now- 

 known to be Laurentian." (L c, A, p. 136). It is well known that in 

 general Dr. Hunt claims a reverse order, or that the Montalban overlies 



the Huronian. 



In 1876 Dr. Hunt referred the ores, found along the borders of the 



Mesozoio red sandstone, and including the Cornwall mine, to the Lower 

 Taconic (Taconian) ; but no evidence was given to show the justness of 

 this conclusion. (Trans. Am. Inst. Miu. Eng., 1876, IV., p. 320.) 



During this year he also referred other rocks in Pennsylvania to the 

 Laurentian, Iluronian, and Montalban. This determination seems to 

 have been based exclusively on lithological characters, no further evi- 

 dence except dili\jrence in the strike of their supposed stratification 

 planes being advanced. The Primal slates and sandstones of Rogers 

 were regarded by Dr, Hunt as Lower Taconic (Taconian). (Proc. Am. 

 Assoc. Adv. Sci., 1876, XXV., pp. 208-212.) He also makes the White 

 Mountain (Montalban) series in the vicinity of Philadelphia to overlie 

 the Iluronian, although Professor Lesley states that the reverse appears 



to be the case. 



la Professor Frederick Prime's Report (Second Geological Survey of 

 Pennsylvania, DD, 1878) the gneissic rocks in Lehigh County arc called 

 Laurentian. In one locality he says that 



"the junction of the Potsdam sandstone and Laurentian rocks can he well 

 seen. The dips of the two rocks seem to be conformable, but this may be 

 wrong, as the exposure ia small and the gneiss apparently has a slight roll. 

 The gneissic rock is here distinctly bedded." 



It is probable that Pi'ofcssor Prime uses the term '* distinctly bedded " 

 for " distinctly foliated," terms which have "'^t as vet been proved to be 

 synonymous. He further says : 



** It is possible 'that these gneissic rocks which seem to lie conformably with 

 the sandstone, and which arc true gneisses, are in reality Lower Potsdam." 

 (I. c, pp. 9, 10.) 



In 1877, Professor Prime advances some evidence to show that the 

 Potsdam sandstone was deposited upon the gneiss and made out of its 

 ruins ; had he deemed this determination a matter of as much impor- 

 tance as it seems to us to be, he would doubtless have investigated the 

 matter more closely. It is to be hoped that this may yet be done. He 

 shows that the mica schists (hydro-mica slates) lie between the Potsdam 



n 



