CONTACT PHENOMENA.—Under this head are grouped the interesting and important phenomena observable along the contact between the dike and wall-rock. These throw light upon the conditions of formation of dikes, and are often depended upon to show whether a rock mass is a dike or not. The student will observe here:—
1. The detailed form of the contact. It may be straight and simple, or exceedingly irregular, the dike penetrating the wall, and enclosing fragments of it, as in Fig. 11, which is a typically igneous contact.
2. The alteration of the wall-rock by heat. This may consist in: (_a_) _coloration_, shales and sandstones being reddened in the same way as when clay is burnt for bricks; (_b_) _baking and induration_, sandstone being converted into quartzite and even jasper; clay, slate, etc., being not only baked to a flinty hardness, but actually vitrified, as in porcelainite; and bituminous coal being converted into natural coke or anthracite; and (_c_) _crystallization_, chalk, and other limestones being changed to marble, and crystals of pyrite, calcite, quartz, etc., being developed in slate, sandstone, and other rocks.
3. The alteration of the dike-rock by (_a_) more rapid cooling, and (_b_) the access of thermal waters.
The alteration of the wall-rock may extend only a few inches or many yards from the dike, gradually diminishing with the distance; and the cases are surprisingly numerous where there is no perceptible alteration; and, again, the alteration is usually mutual, the dike-rock being altered in texture, color, and composition.
[Illustration: Fig. 13.—Ideal cross-section of a laccolite.]
[Illustration: Fig. 14.—Ideal cross-section of a volcano.]
INTRUSIVE BEDS.—We commonly think of dikes as cutting across the strata, but they often lie in planes parallel with them; and the same dike may run across the beds in some parts of its course and between them in others (Fig. 12), or the conformable dike maybe simply a lateral branch of a main vertical dike, as shown in the same figure. All dikes or portions of dikes lying conformably between the strata are called _intrusive beds_ or _sheets_.
When a dike fails to reach the surface, but spreads out horizontally between the strata, forming a thick dome or oven-shaped intrusive bed, the latter is called a _laccolite_ (Fig. 13). Laccolites are sometimes of immense volume, containing several cubic miles of rock. Fig. 14 enables us to compare the laccolite with the volcano.