592 GEOLOGY. 



between beds, it forms sills; bunched under strata so as to arch them 

 upwards, it forms laccoliths; massed in great aggregations undergroimd, 

 it constitutes batholiths, as already described (pp. 394 and 500). Lavas 

 sometimes crowd aside the adjacent rocks so far as to cause them to 

 take a concentric form about the intruded mass. This is not uncommon 

 in the oldest formations, and is probably not infrequent in the deeper 

 horizons where the pressures are very great. Some part of this may, 

 however, be due to later deformations. Nearer the surface, usually, 

 the beds are merely lifted as in forming the sills, or are bowed upwards, 

 as in the laccoliths, or faulted as in hysmaliths (p. 500). 



The heating action on the adj acent rock varies greatly with the mass 

 and temperature of the intruded lava. Thin dikes and sills often 

 produce little effect, while greater and hotter masses notably meta- 

 morphose the adjacent rock. In some cases marked effects are due to 

 a thin stream of lava flowing through a fissure for a long period, and 

 so maintaining a high temperature. In the least effective cases, the 

 adjacent rock usually shows some signs of baking. In the marked 

 cases, there is more or less new crystalhzation. The surrounding rock 

 commonly shows some evidence of material derived from the lavas; less 

 often the lava shows some evidence of having received material from 

 the adjacent rock. But since the lavas do not usually bore their way 

 through the strata in the zone of fracture, nor melt the adjacent rock, 

 the constitution of the lavas is not appreciably changed by the kinds 

 of rock which they penetrate. On the other hand, the intrusions often 

 show the effects of rather rapid cooling by contact with the adjacent 

 rock, (a) by a less coarse crystallization near the rock-walls, and some- 

 times (h) in a segregation of the material. 



2. Extrusions. 



When molten rock is forced to the surface it gives rise to the most 

 intense and impressive of all geological phenomena. The energies 

 acquired in the interior under great compression here find sudden relief. 

 Occluded gases often expand with extreme violence, hurling portions of 

 the lavas to great heights and shattering them into fragments con- 

 stituting ''smoke," ash, cinders, bombs, and other pyroclastic mate- 

 rial. Much of the explosive violence of volcanoes has been attributed 

 to the contact of surface-waters with the hot rising lava, but the function 

 of tliis kind of action has probably been exaggerated. 



