^'^T-2. PETROLOGY AT GOOSE CREEK — SHANNON. 15 



sense in which the term denotes any unusually coarsely crystalline 

 phase of an ordinary fine grained rock where it is necessary, to ac- 

 count for the abrupt change in coarseness of grain, to assume the 

 presence of some special factor, such as a greater concentration of 

 volatile constituents or mineralizers, which has been active in pro- 

 moting the growth of large crystals. The use of the name is not 

 defended against those who favor a rigid confinement of the term peg- 

 matite to its narrower usage to designate macrographic quartz-feldspar 

 mtergrowths or the granitic veins in which such graphic-granite 

 occurs. It is a name which quite naturally suggested itself for the 

 rocks described below and, it is believed, is fairly descriptive, both 

 of their unique texture and of their most probable mode of origin. 

 Similar rocks have previously been described under a variety of 

 names. Emerson has termed coarse phases of diabase in the Triassic 

 of Massachusetts plumose diabase and I have noted a similar rock 

 from the vicinity of Westfield as coarse gabbroid diabase. Bowen 

 called the coarse phases of the Gowganda Lake sills gabbro. Similar 

 variants of the Duluth gabbro, however, have been described by 

 Grout as gabbro pegmatites, and entirely analogous structures in the 

 granite of Quincy, Mass., have been called pegmatites bv Palache 

 and Warren. 



In general, the minerals of the Goose Creek diabase pegmatite are 

 the same as those of the normal diabase into which it grades. The 

 specimen illustrated in plate 2, lower, shows the appearance of a hand 

 specimen trimmed from a typical mass. The most conspicuous mac- 

 roscopic feature of the rock is the augite which forms long bladelike 

 crystals set in a greenish base of feldspathic material. In average 

 occurrences these pyroxene blades range from 4 to 10 cm. long and 

 4 to 8 mm. wide although occasionally much coar«er rock occurs 

 One block found in the woods south of the railroad, where it had been 

 thrown from the quarry by a blast, contained pyroxene crystals up 

 to 20 cm. long and 2 cm. wide. This pyroxene is found, by optical 

 study, to be purplish-brown titaniferous high-iron augite entirely like 

 that of the normal diabase, an analysis of which is given above. Like 

 the pyroxene of the normal rock, these blades show twinning on (100) 

 and pol.ysynthetic twinning and parting on (00 1 ) . In addition to the 

 features which it shares with that of the normal rock, the pyroxene 

 of the diabase pegmatite possesses a well defined parting (diallagic) 

 paraUel to the (100) pinacoid. When the rock is broken this part- 

 mg surface is always exhibited by the augite, and when examined 

 carefully the parting surface shows a bronzy luster with innumerable 

 fine transverse striations which are caused by the trace of the (001) 

 parting and twinning. The blades almost invariably show also a 

 narrow median line which, so far as could be determined, is a narrow 



