ROCK TEXTURES OF CONTACT ZONES. 273 



feature of the plutonic rocks, the contact-zone. This exphiins well why 

 the}^ occur alike around granite and gabbro '^ and produce no marked 

 chemical alteration, as Harker and Wadsworth have observed. A little 

 fluorization may sometimes be noted. On the other hand, I find no 

 record of contact action of porphyries. They have neither the heat nor 

 the Avater. The basaltic rocks show almost purely the action of heat at 

 their baked, indurated and semi-fused contacts, and better than in any 

 other way the heterogeneit)'' of the rocks which have been called diabases 

 is shown by the variety of contact-zones reported. On the strength of 

 these alone I should expel some to the melaphy re-basalt family, class 

 some with the gabbros, regardless of diallagic cleavage, and maintain the 

 narre for a distinct group occurring characteristically in dikes and in- 

 trusive sheets. 



Textures of Rocks. — These gases ma}'' also remain in the magma through- 

 out crystallization. In such cases there will be interstices left, giving us 

 the miarolitic texture, with which the panidiomorphic textures are closely 

 akin. In the last stages of cooling, when there are but small residual 

 spaces left, they would doubtless be filled with mineralized Avater. The 

 crystallization thereafter belongs to what Brogger has called the pneu- 

 matoh^tic stage, and microline, muscovite, calcite and quartz are some of 

 the proba])le products. What I have described as the acid interstices in 

 quartz-diabases, which in their intergrowths of quartz and felspar in as- 

 sociation with hornblende and biotite remind one at once of acid rocks, 

 seem to have been thus found. Lawson has described similar things 

 from the north shore of I.ake Superior. He moreover added the im- 

 portant observation that they increase and the rock becomes more acid 

 toward the center of the dike. This leads naturally to the third disposi- 

 tion ot the gases that can be thought of; that is, that they may be con- 

 centrated into fissures and cavities in the rock. The case of the coarse 

 mottled melaphyres of Isle Royale, with the streaks here and there of a 

 more gabbroid texture, shows the beginning of this process. 



Any one who has studied a granite area in the field must, I think, 

 have been struck with the difficulty of separating segregation veins from 

 pegmatites on the one hand, and pegmatites from the normal granite on 

 the other.f Very significant in this connection is the change in position 

 of Professor Rosenbusch, who used to class pegmatites with veins, but 

 now places them with igneous rocks. Such an authority must have had 

 good reasons both for the position he formerly had and that which he 

 now takes. The explanation is simple. The pegmatites are one form of 

 residual magmas, the segregation veins a farther step. Brogger 's work 



*Bayley has described a case of the latter at Pigeon point. Am. Jour. Soi., vol, xxxix, 1890, p. 273. 

 fOeikie: Op. cit., p. 700. 



