Page:The Journal of geology (IA journalofgeology11893univers).pdf/419
fixed for similar kinds of rocks that it would be almost impossible 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 at 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 composition 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 abundant 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 transitional 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 occurrences 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 noticeable 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.