SEEIES OF INTEUSIVES. 265 



a constant diminution corresponding' to the increasing magnesian character 

 of the rocks. The potash increases as the soda shows a decrease. 



The rocks represented by the analyses are beheved to belong to a 

 series ranging from a diorite on the one hand, through hornblende-gabbro 

 and norite, to peridotite on the other. It should be borne in mind that the 

 diorite is somewhat exceptional, representing a gradation toward the ortho- 

 clase rocks. On the acid side of the series the microscope also shows 

 variations to tonalitic and even granitic rocks very rich in quartz and ortho- 

 clase, consequently much more acid in character than the diorite repre- 

 sented in the analysis. ■ 



It is a difficult matter to estimate quantitatively the amount of the one 

 or the other kind of rock present in the Crystal Falls district. We are thus 

 prevented from drawing from the predominance of the one kind or the 

 other the conclusion that those represented in the minority are the results 

 of the differentiation of a magma most nearly resembling- in its original 

 ' constitution that which predominates. Moreover, since the analyzed rock 

 types were not selected as representatives of the extremes of the process of 

 differentiation, it would not be wise to endeavor to give the mean composi- 

 tion of the parent magma from the analyses of the differentiation products 

 which' have been presented. The main thesis, however, is established that 

 the separation of a magma into the various products described has taken 

 place, as is indicated by the relations in the field, and as has been shown by 

 the microscopical and chemical analyses. 



RELATIVE AGES OF ROCKS OF THE SERIES. 



Study of the relative periods of eruption of the various rocks results in 'the 

 determination of the hornblende-gabbro as the rock which first reached its 

 present position. It was followed in the acid part of the series by the diorite, 

 which in one place cuts it. The diorite is cut by the diorite-porphyry. 



Along the basic series the order has been determined as hornblende- 

 gabbro, gaibbro, bronzite-norite, peridotite. 



In general the forces of differentiation seem to have been active in two 

 directions, tending toward increasing acidity and increasing basicity of the 

 products of differentiation, thus agreeing with the law of succession of 

 igneous rocks as propounded by Iddings.^ 



• The origin of igneous rocks, by J. P. Iddiugs: Bull. Philos. See. Wash., Vol. XII, 1892, p. 195. 



