DIORITE INTEUSIVES. 227 



with a maximum of 15 degrees. Only one kind of plagioclase is distin- 

 guishable b J its mode of development, and this is rich in CaO, as shown by 

 its alteration products. The feldspar ranges at most from albite to andesine. 

 No chemical analysis has been obtained of either the granitite or the mica- 

 diorite phase, but the mineralogical composition is sufficiently marked to 

 show conclusively that we have here a gradation from a g-ranitic to a dioritic- 

 rock rich in CaO. The idea has been suggested by Johnston-Lavis ^ that 

 in some cases the variation in chemical composition of intrusive rocks, 

 especially where this variation is one between the center and the jjeriphery 

 of an intrusive mass, may be due to resorption by the intrusive of parts of 

 the rock intruded. The sharp line of demarcation which exists between 

 the dike and the intruded hornblende-gabbro in the occurrence described 

 above seems to preclude the possibility of a fusion and mingling of the two 

 rocks. 



ACROSS KIVER FROM CRYSTAL FALLS. 



Near Crystal Falls, just across the river from the town, are a number 

 of small knobs of granite grading into quartz-mica-diorite. They are 

 medium-grained rocks, reddish to gray in color. They take a very fine 

 polish and are well adapted to ornamental stonework, as is shown by the 

 columns made from thein which are used in the court-house at Crystal 

 Falls. When examined under the microscope, the rocks are found to con- 

 sist of autoinorphic biotite and plagioclase, with xenomorphic orthoclase 

 and quartz, these last forming the cement. Some of the slides show 

 beautiful micropegmatitic intergrowths of quartz and feldspar. The amount 

 of quartz, plagioclase, and orthoclase varies so that, depending upon the 

 specimen examined, one would call the rocks forming the knobs granite or 

 quartz-mica-diorite. Most commonly the rock is a plagioclase-bearing- 

 granite. No analysis has been obtained of the granite, but it is confidently 

 believed that the chemical composition would sustain the microscopical 

 diagnosis. Within the granite there are found lenticular schlieren of 

 considerably darker color than the main mass, in which the plagioclase is 

 the preponderant feldspathic constituent. The rock of these lenses is 

 essentially a quartz-mica-diorite. 



'The basic eruptive rocks of Gran (Norway) and their interpretation; a criticism by H. J. 

 Johnston-Lavis: Geol. Mag., 4th ser., Vol.1, 1894, p. 252. 



The causes of variation in the composition of igneous roclis, by H. J. Johnston-Lavis: Natural 

 Science, Vol. IV, 1894, pp. 134-140. 



