18 J. Barrell — Relations of Subjacent Igneous 



showing all gradations from quartz to pure tremolite. 

 The tremolite veins in places cut the older vein quartz 

 and give clear evidence of the carrying of tremolite in 

 solution. 



Between Ashley Falls and Falls Village another phase 

 may be seen. Tough, massive reefs of white pyroxene 

 rock, malacolite, are found, with granitic grain and in 

 some places clearly developed by infiltration, not by the 

 mere alteration of an impure marble. The reefs occur in 

 certain local areas and are not restricted to a definite 

 horizon in the marble. The large size and unbroken 

 character of the crystals, as well as the presence of 

 pyroxene rather than amphibole, ally these occurrences 

 with the phenomena of igneous contact action, and show 

 a development after the regional mashing. In general, 

 these reefs are surprisingly free from the minerals 

 usually associated with igneous emanations. Only rarely 

 is pyrite or chalcopyrite found, and in only one locality 

 near the eastern margin of the limestone is tourmaline 

 associated. Here, near the head of A^^iiting River, large 

 crystals of feldspar and phlogopite and smaller ones of 

 tourmaline occur with malacolite. Thus this evidence, 

 while indicative of rising waters, does not clearly show 

 those waters to have been magmatic. 



Lack of Metamorphism in the Pennsylvania Folds. 



The valley in Pennsylvania between the pre-Cambrian 

 and Silurian outcrops corresponds in structural position 

 to the valley in western New England which has just been 

 described. It is a region of close folding, the folds be- 

 coming open farther west in the state. The limestones 

 of this eastern belt have been crumpled into folds, many 

 of which are but some tens of feet in radius. In other 

 places, extensive overturning into recumbent folds has 

 taken place. The argillites above have been mashed into 

 slates holding much of commercial quality. The w^riter 

 has recently assembled the evidence to show that a great 

 depth of cover has been removed from this region,^^ but 

 the nature of the structures alone serves to make it clear 

 that they originated at considerable depth. It is re- 

 garded as a tract of great crustal shortening,^^ and the 



^° Joseph Barrell, The Upper Devonian delta of the Appalachian geosyn- 

 cline, this Journal (4), 36, 429-472, 1913; 37, 87-109, 225-253, 1914. 

 ^^ E. T. Chamberlin, Jour. Geology, 18, 228-251, 1910. 



