CRYSTALLIZATION OF DIKE ROOKS. 



107 



to the biotite in the diorite. The gradual nature of the transition from oue 

 extreme to the other is apparent at a glance. 



The impossibility of considering certain rocks as definite types with 

 which to compare other rocks in the region is also evident when it is 

 observed that the mineralogical variation takes place within certain limits 

 in one rock body (Nos. 247, 250, 255, 256, and 257 are from the same 

 dike), and that what appears to be a mineralogical modification of one 

 particular rock body is the characteristic combination of another, and its 

 modification is something different. Field observation shows that in this 

 locality the greater number of dikes are composed of rocks with variable 

 percentages of porphyritical hornblende and biotite, and that the other 

 varieties are less numerous. In another region other varieties predominate. 

 The chemical variations which are indicated by the silica percentages 

 range from 57.12 in subdivision d 3 to 61.85 in cl 7 , and jjrobably reach 69.00 

 in d n . They indicate a correspondence between the mineralogical and 

 chemical variations for this group of rocks. 



Table IV. — Grades of crystallisation of the dike rocks of Electric Peak. 



Grades 

 of crystal- 

 lization. 



Mineralogical grouping indicated in Table III. 



d x 



d 2 



d 3 



<*4 



di 



<?6 



d, 



<?» 



d, 



d,a 



dn 



6 























259 

 260, 261 

 262, 263 

 264, 265 



7 













240 











8 











238 











9.. 











241, 242 



243 



244, 245 









258 



10 













246 







11 















255, 256 

 257 







12 











239 





249, 250 

 251 



252, 253 

 254 







13 



















14 















247, 248 









16 







235 















19 



20 



25 



232 

 233 





236, 237 

































234 











































Table IV expresses the range in degree of crystallization of the ground- 

 mass of these rocks, which are arranged in columns corresponding to the 

 mineralogical grouping of Table III. It is to be remarked that the speci- 

 mens were collected from different-sized dikes and from different parts of 

 the dikes, so that the variations in grain can not be compared very closely 



