SOME EXAMPLES OF ROCK VARIATION $77 



occurrence from the one just described, in which the greater 

 portion of the rock is a plagioclase-bearing biotite-granite, show- 

 ing in many cases most beautiful micropegmatitic texture, but 

 with schlieren of a darker colored rock which might readily be 

 called a quartz-mica-diorite. It is especially interesting to note 

 also that this series of exposures is cut by a number of small 

 dikes of uniform character, varying from fractions of an inch to 

 three inches in width. The dike rock is very light gray to pink 

 in color, and cryptocrystalline. Examined under the microscope, 

 these dikes can be readily separated into a microporphyritic 

 center, and a microgranitic textured selvage. Phenocrysts of 

 quartz, feldspar, and biotite lie in a microgranitic groundmass 

 of feldspar and quartz, between which occur secondary flakes of 

 muscovite. The rock is a microporphyritic quartz-mica-diorite 

 or quartz-mica-diorite-porphyry. This rock seems to bear a 

 very strong resemblance to the tonalite-porphyrite described by 

 Becke * which occurs as a dike facies of the Rieserferner tonalite 

 to which I have already referred. 



Still another illustration of the passage from the granitic to 

 the dioritic rocks was observed upon - a dike, four feet wide, 

 which penetrates a knob of hornblende-gabbro. A specimen 

 taken near the center of the dike discloses itself as a biotite- 

 granite with a small amount of plagioclase. The sides of the 

 dike consist of diorite, composed of andesine feldspar, and 

 biotite, without any quartz. The sharp line of demarkation 

 which exists between the dike and the gabbro seems to pre- 

 clude the possibility of a fusion and mingling of the two rocks, 

 such as has been suggested by Johnston-Lavis, 2 as in some 

 cases causing variation in chemical composition of intrusive 

 rocks, especially where this variation is one between the center 

 and periphery of an intrusive mass. The diorite occurrences of 

 the Crystal Falls district seem, in the gradations mentioned, to 



1 Loc. cit., pp. 434-441. 



2 The causes of variation in the composition of igneous rocks, by H. J. Johnston- 

 Lavis : Natural Science, No. 4, pp. 134-146. 



The basic eruptive rocks of Gran, Norway, and their interpretation, by H. J. 

 Johnston-Lavis : Geol. Mag., 4th decade, Vol. I, 1894, p. 252. 



