VOLCANOES AND IGNEOUS INTRUSIONS 



327 



Laccoliths (Greek, lakkos, a cistern, and lithos, stone). — This tern? 

 has been given to mushroom-shaped intrusions of lava which have 

 been forced along bedding planes and have domed up the overlying 

 strata. They are 

 formed when molten 

 rock rising through 

 a pipe or fissure is 

 unable to break 

 through the overly- 

 ing rock and spreads 

 between the strata, 

 lifting them and thus 

 producing domelike 

 elevations (Figs. 324, 

 325). The difference 



, .], 1 Fig. 324. — Diagram illustrating the form and relations 



between a Sill and a f dikes, A and D; sills, C and E; and a laccolith, B. 

 laccolith is conse- 

 quently a difference in the degree of the doming of the overlying 

 strata. Laccoliths may be a mile or more thick and a number of 

 miles in diameter. Mountains of considerable height have been 

 formed in this way. The Henry Mountains of southern Utah, 

 the Elk Mountains of Colorado, and many other elevations in the 



Rocky Mountains 



are laccoliths (Fig. 



325). Laccoliths are 



composed of lava 



which was probably 



stiff and viscous and 



could consequently 



more easily lift the 



strata than force its way between them. The lavas of sills, on the 



other hand, were probably quite fluid and therefore could spread 



long distances. 



II. Subjacent Masses 



Stocks. — The name stock is applied to large bodies of igneous rock 

 lying in the midst of other formations. Stocks are usually circular or 

 elliptical in outline and vary from a few hundred yards to many 

 square miles in extent, usually increasing in size downward (Fig. 

 326 J, B). Since they are composed of more resistant rock than 



Fig. 325. — Diagram of a laccolith, showing the rela- 

 tion of the igneous intrusion to the overlying and under- 

 lying strata. 



