On Composite Dykes in Arran

Abstract

I. Definition and Classification of Composite Dykes. Those dykes which are filled with materials differing one from another in chemical composition or mineralogical constitution may be conveniently spoken of as ‘composite dykes.’ In some cases the diverse rocks found in the same dyke may be only varieties of a single type, exhibiting dissimilarities in the proportions or arrangement of their several mineral constituents; but in other cases two or more rock-species, absolutely distinct, may be found to enter into the constitution of a single dyke. A careful study of these composite dykes shows that they fall naturally into two classes, as follows :— (A) Composite dykes in which a differentiation has gone on in the material that has filled the dyke .—This differentiation usually results in the separation, or partial separation, of the vitreous base of the rock from the crystals scattered through it ; but in some cases the crystallizing process, acting selectively within the dyke, has led to the formation of rocks which, in different parts of the dyke, show marked variations in chemical composition or texture, and even in mineralogical constitution. The separation of the glassy from the crystalline materials in this variety of composite dykes takes place in various ways ; the vitreous material may form lateral bands (‘ selvages ’) or irregular patches, or it may fill ‘ amygdaloidal’ cavities. But in all cases there is found to be a more or less perfect gradation from the one type of rock to the other. (B) Composite