When the molten magmas escape to the surface, they are termed extrusive, and as they spread out in a layer this is termed a sheet. This rise and overflow may be quiet, and from time to time one outpouring may follow another making sheet after sheet. Or after one outpouring, the pressure below may cease for a time and allow the lava to solidify and make a cap or cover over the opening. Before more lava can rise, this cover must be removed. This usually happens in an explosive manner, the lava below, with the increasing pressure exerted by its expanding gases, finally exerting enough pressure, so that the cover is broken, or shattered and thrown in thousands of fragments into the air, as happened at Mt. Pelée on the Island of Martinique in 1902. The fragments thrown into the air are often termed volcanic ashes, though this is not a good word for them, for they have not been burned. In case the molten magmas under the relieved areas do not reach the surface they are termed intrusive. Such magmas may remain in the space under a mountain fold, or be forced in fissures part way to the surface. When the magma is forced into more or less vertical cracks and there solidifies, and these are exposed by erosion, they are termed dikes. Sometimes the magmas have risen part way to the surface and then pushed their way between two horizontal layers of rock and there hardened, in which case they are termed sills, when uncovered. The Palisades along the Hudson River are the exposed edge of a sill. Again the molten magmas may well up and spread between two horizontal layers, but come faster than they can spread horizontally, and then the magma takes the form of a half sphere, and the overlying layers of rock are domed up over it. Such a mass is termed a laccolith. In all these cases the mass of igneous rock is only discovered when the overlying rocks have been eroded off. The great mass of molten magma under the arches of mountain ranges simply cools slowly into a granitic type of rock. These masses are exposed when the thousands of feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles in extent, they are called stocks, but, if of many miles in length and breadth, they are batholiths, and are very characteristic of the heart of mountain ranges.