There are two kinds of diabase known in the district, one of which greatly exceeds the other in extent and economic importance. This disparity is tacitly recognized by the common use of the generic term diabase for the important quartz diabase, the other member of the family being ignored or unknown. _Quartz Diabase._ [Illustration: Fig. 7. Inclined Huronian Beds, Duncan Lake.] _Character of contacts and distribution._—Bodies of this rock are not restricted to any one formation or locality; but, as may be seen by reference to the map, occur with various dimensions over the whole area, with the exception of the country lying north-west of Duncan lake. It may be noted that virtually all the large areas are enclosed by Huronian, the bodies within the Archæan being numerous but small. It is not proposed to apply this distinction generally, but in the Montreal River region it seems to be more than an expression of the law of probability. In the Archæan practically all of the observed diabase bodies are dikes, in approximately vertical positions, seldom 100 feet in width and of undetermined length. A knowledge of all those in the Huronian could not be obtained, but in some cases they are sills, whose greater exposed dimensions are parallel to the bedding planes of the enclosing sedimentaries. _Macroscopic character._—It is fortunate for the easy recognition of the diabase that it is more or less continuously exposed, and that there are not many other igneous rocks of similar character in its proximity, for it presents a variety of types such that a representative collection of hand specimens presents surprisingly great petrological differences. During the process of solidification, magmatic differentiation evolved from the cooling material a group of forms of different mineralogical composition and physical appearance, the end members of which are very unlike. The common type is a dark green massive diabase, ranging in texture from one in which the individual minerals are barely distinguishable to others containing amphibole crystals an inch in length. The combination of black amphibole—an alteration product of augite—and grey or flesh-coloured plagioclase give the surface of ordinary textured phases a colouration of sufficient determinative value. In a large diabase body the coarseness is equal to that of ordinary granite, and the diabasic structure is not readily perceptible. At the edge of the large dikes and throughout the smaller ones the rock is black in colour and much finer grained on account of its more rapid solidification.