=Intrusive sheets.= Sheets of igneous rocks are sometimes found interleaved with sedimentary strata, especially in regions where the rocks have been deformed and have suffered from volcanic action. In some instances such a sheet is seen to be _contemporaneous_ (p. 248). In other instances the sheet must be _intrusive_. The overlying stratum, as well as that beneath, has been affected by the heat of the once molten rock. We infer that the igneous rock when in a molten state was forced between the strata, much as a card may be pushed between the leaves of a closed book. The liquid wedged its way between the layers, lifting those above to make room for itself. The source of the intrusive sheet may often be traced to some dike (known therefore as the _feeding dike_), or to some mass of igneous rock. Intrusive sheets may extend a score and more of miles, and, like the longest surface flows, the most extensive sheets consist of the more fusible and fluid lavas,--those of the basic class of which basalt is an example. Intrusive sheets are usually harder than the strata in which they lie and are therefore often left in relief after long denudation of the region (Fig. 315). [Illustration: Fig. 243. Diagram of the Palisades of the Hudson _i_, intrusive sheet; _s_, sandstone; _d_, feeding dike; _HR_, Hudson River] On the west bank of the Hudson there extends from New York Bay north for thirty miles a bold cliff several hundred feet high,--the _Palisades of the Hudson_. It is the outcropping edge of a sheet of ancient igneous rock, which rests on stratified sandstones and is overlain by strata of the same series. Sandstones and lava sheet together dip gently to the west and the latter disappears from view two miles back from the river. It is an interesting question whether the Palisades sheet is _contemporaneous_ or _intrusive_. Was it outpoured on the sandstones beneath it when they formed the floor of the sea, and covered forthwith by the sediments of the strata above, or was it intruded among these beds at a later date? [Illustration: Fig. 244. Section of Electric Peak. E. and Gray Peak, G, Yellowstone National Park Intrusive sheets and masses of igneous rock are drawn in black] The latter is the case: for the overlying stratum is intensely baked along the zone of contact. At the west edge of the sheet is found the dike in which the lava rose to force its way far and wide between the strata. _Electric Peak_, one of the prominent mountains of the Yellowstone National Park, is carved out of a mass of strata into which many sheets of molten rock have been intruded. The western summit consists of such a sheet several hundred feet thick. Studying the section of Figure 244, what inference do you draw as to the source of these intrusive sheets? [Illustration: Fig. 245. Stone Mountain, Georgia, a Granite Boss] Intrusive Masses