THE IGNEOUS liOCKS. 497 



rocks, and sometimes there are present small nests of secondar}- (juartz. 

 All plagioclase has entirely disappeared. The forms of its crystals are 

 preserved by calcite and epidote, or perhaps by calcite alone, which pseudo- 

 morphs the feldspar. 



Where rendered schistose by mashing, as happens on the peripheries 

 of most of the knobs, the alteration of the greenstones is far advanced. 

 Chlorite and calcite, with a little magnetite, quartz, and other minerals in 

 small quantity, sometimes constitute the entire rock, which is then a typical 

 chlorite-schist. In no cases observed are the greenstone-schists enveloping 

 the more massive greenstones of sedimentary origin, as might be inferred to 

 be the case from Wadsworth's^ earlier statements. In all cases the schistose 

 rocks differ from the more massive ones in being more highly foliated through 

 mashing, which was naturally more easily possible on the peripheries of the 

 rock masses than elsewhere. The schists often exhibit traces of the diabasic 

 structure, even when greatly altered. The degree of alteration to which 

 they have been subjected appears, however, to have increased with the 

 degree of the foliation, so that the most schistose of the peripheral rocks 

 have lost all traces of their origin. It is principally through their gradation 

 into less highly foliated phases that their true nature is made out. 



Under the microscope the foliation of the schists is plainly seen to be 

 <in effect of motion in a solid rock mass. Broken crystals of plagioclase, 

 cr3'stals faulted along cracks, others crushed into powder on their borders, 

 and others fissured, with their cracks filled with a secondar}- mosaic like 

 that of the rock's groundmass, all bear strong evidence that the rocks in 

 which these phenomena are found have been at some time subjected to 

 great stresses. The foliation is due to the arrangement of the chlorite and 

 amphibole in parallel libers, and, since the direction of the parallelism cor- 

 responds with that along which the particles of the liroken crystals have 

 been moved, it is concluded that the schistosity is also an effect of pressure. 



While the rocks described above are the predominant ones in the 

 eastern knobs, there is another type that should be mentioned. A consid- 

 erable number of the greenstones are dark-green in color and coarse- 

 grained. On a freshly fractured surface brilliant black columnar crystals of 



' Bull. Mu8. Comp. Zool. Harvard Coll., Geol. Series, No. 1, 1880, p. 41. 

 MON XXVIII 32 



