These are horizontal or moderately inclined masses of igneous rock, which have small thickness as compared with their lateral extent. Sheets conform to the bedding-planes of the strata, often running long distances between the same two beds; but if they can be traced far enough, they may generally be found cutting across the strata at one point or another. In thickness they vary from a few feet to several hundreds of feet. The Palisades of the Hudson are formed by a sheet of unusual thickness; its outcrop is 70 miles long from north to south, and its thickness varies from 300 to 850 feet.

Sills are most commonly found in horizontal strata, which offer less resistance to horizontal expansion than do the folded beds; they are also very generally of the most fusible kind, the gabbro family, because such magmas retain their fluidity and flow for longer distances than do the highly siliceous rocks. It is probable that intrusive sheets can be formed at only moderate depths, because the overlying strata must be lifted to an amount equal to the thickness of the sheet, although certain cases are known where the sill appears to have made its way by melting and incorporating some of the strata. At great depths the weight to be lifted is so enormous, that the easiest path of escape must be by breaking through and across the strata. If the beds are subjected to compression after the intrusion of the igneous masses, the latter will be flexed or faulted like the stratified rocks.

Granite veins intrusive in diorite and both cut by a small dyke of aplite; coast of Maine.

Fig. 213. - Granite veins intrusive in diorite and both cut by a small dyke of aplite; coast of Maine. (U. S. G. S).

In a limited exposure it is often difficult to distinguish at once between a sill and a contemporaneous sheet, but there are certain characteristic marks which enable the observer to decide. The presence of scoriae shows that the sheet is contemporaneous. If, on the other hand, the overlying stratum be baked and altered by the heat, or if the sheet cuts across the bedding-planes at any point, or if it can be traced to a dyke which rises above it, or if it gives off tongues or veins, or if pieces of the overlying stratum be torn off and included in the sheet, it must be intrusive. The nature of the contact between the sheet and the stratum above it • is also significant; if the former be contemporaneous, the cracks and fissures of its upper surface will be filled with the sedimentary material. Finally, the texture of the igneous mass gives valuable evidence; in the intrusive sheet the texture is compact (without glassy ground mass) or even quite coarsely crystalline, while the contemporaneous sheet will display the glassy or por-phyritic texture of surface flows.

The Palisades, seen from Hastings, N.Y. (Photograph by van Ingen).

Fig. 214. - The Palisades, seen from Hastings, N.Y. (Photograph by van Ingen).

Contact of diabase sill with shales below. Base of Palisades, Weehawken, N.J. (U. S. G. S).

Fig. 215. - Contact of diabase sill with shales below. Base of Palisades, Weehawken, N.J. (U. S. G. S).