Dikes.  The sheet of once molten rock with which a fissure has been filled is known as a dike.  Dikes are formed when volcanic cones are rent by explosions or by the weight of the lava column in the duct, and on the dissection of the pile they appear as radiating vertical ribs cutting across the layers of lava and tuff of which the cone is built.  In regions undergoing deformation rocks lying deep below the ground are often broken and the fissures are filled with molten rock from beneath, which finds no outlet to the surface.  Such dikes are common in areas of the most ancient rocks, which have been brought to light by long erosion.

In exceptional cases dikes may reach the length of fifty or one hundred miles.  They vary in width from a fraction of a foot to even as much as three hundred feet.

Dikes are commonly more fine of grain on the sides than in the center, and may have a glassy and crackled surface where they meet the inclosing rock.  Can you account for this on any principle which you have learned?

Volcanic necks.  The pipe of a volcano rises from far below the base of the cone,—­from the deep reservoir from which its eruptions are supplied.  When the volcano has become extinct this great tube remains filled with hardened lava.  It forms a cylindrical core of solid rock, except for some distance below the ancient crater, where it may contain a mass of fragments which had fallen back into the chimney after being hurled into the air.

As the mountain is worn down, this central column known as the volcanic neck is left standing as a conical hill (Fig. 240).  Even when every other trace of the volcano has been swept away, erosion will not have passed below this great stalk on which the volcano was borne as a fiery flower whose site it remains to mark.  In volcanic regions of deep denudation volcanic necks rise solitary and abrupt from the surrounding country as dome-shaped hills.  They are marked features in the landscape in parts of Scotland and in the St. Lawrence valley about Montreal (Fig. 241).

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).