GENETIC RELA TIONSHIPS AMONG IGNEO US ROCKS. 835 



fixed for similar kinds of rocks that it would be almost impos- 

 sible to find two instances in which the proportions between the 

 essential ingredients were exactly the same. The independence 

 of many kinds of igneous rocks might seem a.t first thought to 

 be clearly established by these mineralogical and chemical 

 divergences. This apparent independence disappears when a 

 great number of rocks are investigated. It is found that few 

 rocks contain the same minerals in any given proportion, and 

 that the variable proportions of minerals produce varieties of 

 rocks which grade insensibly from one extreme of mineral com- 

 position into another. Intermediate varieties of rocks which 

 form transitions from one type, or distinct kind, to another have 

 been recognized for many years. But it is becoming more and 

 more evident that the so-called type-rocks are not more abund- 

 ant in nature than the intermediate forms. It is found that 

 particular kinds of rocks may preponderate in one region and 

 the intermediate varieties be subordinate, but that in other 

 localities the relations may be reversed, and the so-called transi- 

 tional forms may prevail. 



The mineralogical gradation of one kind of rock into another 

 is indicated not only by the comparison of all known varieties of 

 igneous rocks, but more especially by the study of all the occur- 

 rences of such rocks in any region where they are abundant. 

 The absence of distinctive types, and the presence of all possible 

 varieties intermediate between the extremes is the most notice- 

 able characteristic. Moreover, the transitional variations are not 

 simply represented by slightly different bodies of rock, but they 

 may often be found to exist within one continuous rock mass. 

 Thus, a large body of rock may change in mineral composition 

 from one spot to another by the most gradual transitions, giving 

 rise to constitutional facies of the main mass. Again, it is found 

 that a large body of rock, which may be nearly homogeneous 

 throughout, exhibits certain mineralogical facies which are like 

 the main portion of some other rock-body in the same region ; 

 so that the subordinate variety in one mass is the predominant 

 form in another. 



