While the diabase dikes are most prominent in the Newark rocks, they are also found occasionally in the other terraces. In the Catoctin Belt they appear irregularly in the granite and schist. Rare cases also occur in the rocks of the Piedmont plain. The diabase of the Newark areas is almost exclusively confined to the red sandstone, and the dike at Leesburg cutting the limestone conglomerate is almost the only occurrence of that combination. The diabase occurs only as an intrusive rock in the vicinity of the Catoctin Belt. Of this form of occurrence, however, there are two types, dikes and massive sheets or masses. The dikes are parallel to the strike of the inclosing sandstone as a rule, and appear to have their courses controlled by it on account of their small bulk. The large masses break at random across the sandstone in the most eccentric fashion. No dislocation can be detected in the sandstones, either in strike or dip, yet of course it must exist by at least the thickness of the intrusive mass. That this thickness is considerable is shown by the coarseness of the larger trap masses, which could occur only in bodies of considerable size, and also by the width of their outcrops in the westward dipping sandstones. The chief mass in point of size is three miles wide. This mass fast decreases in width as it goes north, without losing much of its coarseness, and ends in Leesburg in a hooked curve. The outline of the diabase is suggestive of the flexed trap sheets of more northern regions, but this appearance is deceptive, since the diabase breaks directly across both red sandstone and limestone conglomerate, which have a constant north and south strike. An eastern branch of this mass crosses the Potomac as a small dike and passes north into Pennsylvania. The diabase dikes in the Catoctin Belt are always narrow, and, while many outcrops occur along a given line, it is probable that they are not continuous. At Leesburg the limestone conglomerate next the diabase is indurated, its iron oxide is driven off, and the limestone partly crystallized into marble. _Catoctin Schist._ The Catoctin schist is geographically the most important of the volcanic rocks of Loudoun.