54. Intrusive igneous rocks occur as _sheets_, _dykes_, and _necks_. The sheets frequently conform for long distances to the bedding of the strata among which they occur, and are thus liable to be mistaken for contemporaneous rocks. But when they are closely examined, it will be seen that they not only bake or alter the beds above and below them, but seldom keep precisely to one horizon or level--occasionally rising to a higher, or sinking to a lower position in the strata, as shewn in the following diagram-section. Dykes are wall-like masses of igneous strata which cut across the strata, generally at a high angle (see _d, d_, fig. 22). In the neighbourhood of a recent volcanic orifice, numerous dykes are seen ramifying in all directions. In the British Islands some dykes have been followed in a linear direction for very long distances. Sometimes these occupy the sites of large dislocations, at other times they have cut through the strata without displacing them. Occasionally they appear to have been the feeders of the great sheets of igneous rock which here and there occur in their vicinity. The phenomena presented by the _necks_ of intrusive rock do not differ from those characteristic of _agglomerate_ or _tuff necks_. The strata are bent down towards the central plug of igneous rock, and are generally more or less altered at the line of junction. [Illustration: Fig. 22.--Intrusive Sheet and Dykes: _i_, igneous intrusive sheet; _d_, _d_, dykes; _s_, _s_, sedimentary strata.] 55. Intrusive rocks offer, as a rule, some contrasts in texture to contemporaneous masses. They are seldom amygdaloidal, but when they are so it is generally towards the centre of the mass. The kernels are usually minute and more or less spherical. [Illustration: Fig. 23.--Contemporaneous and Intrusive Igneous Rocks: _c_, _c_, contemporaneous trap-rocks[1]; _t_, _t_, contemporaneous fragmental igneous rocks; _i_, _p_, _n_, _d_, intrusive igneous rocks.] The diagram (fig. 23) shews the general mode of occurrence of igneous rocks on the large scale. The stratified aqueous deposits are indicated at _a_, _a_. These are overlaid by a series of alternating beds of crystalline (_c_) and fragmental (_t_) igneous rocks. An irregular intrusive sheet at _i_ cuts across the beds _a_, _a_. At _p_, another intrusive mass is seen rising in a pipe, as it were, and overflowing the beds _a_, _a_, so as to form a cap. A volcanic neck filled with angular stones intersects the strata at _n_, and two dykes, approaching the vertical, traverse the bedded rocks at _d_, _d_. It will be noticed that the contemporaneous igneous rocks form a series of escarpments rising one above the other.