DIABASE DIKES AND STOCKS IN THE (INEISS. 417 



is seen by the fac-t that the clear nn<>-s pdlarize teel)ly and show traces (if a 

 bhxck cross. 



On the upper side, i. e., opposite the bhick bonh'r, tlic licpiid rock 

 forced its way in several places between the grains of the l)oundin<>- rock. 

 In one place it floAved in with a width of 0.5""", showing a delicate fluidal 

 structure, the lines of black dust being drawn into a series of regular par- 

 abolas, exactly as in a diagram of the surface flow of a river around a 

 curve. Another, narrower, runs far into the gneiss and passes lengthwise 

 of a large biotite crystal in a gliding plane, with a width of O.O'i""". 



The contact effects of the small dike on the gneiss are also interesting. 

 Not only is the former filled with minute fragments of the inclosing rock, 

 as already noted, but in places along the side is finely crushed and dis- 

 turbed, and cemented again by eruptive material. Crystals of triclinic 

 feldspar have their laminae interrupted and echeloned by a series of fine 

 faults, and in the immediate neighborhood of the dikes they were so influ- 

 enced b}- heat that the laminaj, instead of being as usual (and as the}' are 

 here farther away) perfectly straight and sharply defined in polai-ized light, 

 become Avavy and bend over into tlie <lirection of flow of the lava, and the 

 bauds of color pass gradually into each other. 



In other cases, in a feldspar apparently fresh, on approaching extinc- 

 tion a band of black passes in from the border to the center and disappears. 



The large biotite, through which the narrow vein passed, seemed 

 entirely fresh, but in polarized light it was seen to be markedly affected, 

 apparently by compression, so that it broke up into ])atches of color, 

 arranged along the sides of the intruding vein. Smaller crystals of biotite 

 were twisted, so as to show a brilliant aggregate polarization in long inter- 

 woven lilies. 



The large quartz grains, usually entirely uniform, were broken up 

 into irregular patches of brilliant color, and sliowed marked undulatory 

 exthiction. 



Specially rine cabinet specimens of the small dikes mentioned above 

 can he at times obtained from the Monson quarry — hand specimens of the 

 light-gray gneiss, with tlu'ee or four dikes narrower than one's finger cross- 

 ing them, and at times bending round so sharply as to inclose a thin wedge 

 of the gneiss, thinner even than the small dikes themselves. 



From the aphanitic border of the largest dike there, which is only 

 MON XXIX 27 



