LACCOLITHS 



283 



(without glassy ground mass) or even quite coarsely crystalline, 

 while the contemporaneous sheet will display the glassy or por- 

 phyritic texture of surface flows. 



Laccoliths. — A laccolith (or laccolite) is a large, lenticular 

 mass of igneous rock, filling a chamber which it has made for 

 itself by lifting the overlying strata into a dome-like shape. The 

 rock of which laccoliths are made is nearly always of the highly 

 siliceous and less fusible kinds, so that it can more easily lift the 

 strata than force its way between them. Intrusive sheets are, it 

 is true, often given off from a laccolith, but these are of quite sub- 



FlG. 127. — Diagram of uneroded laccolith. (Modified from Gilbert.) 



ordinate importance, while dikes and irregular protrusions extend 

 into the fissures of the surrounding and overlying strata. Sub- 

 sequent erosion may remove the dome of strata and cut deeply 

 into the igneous mass beneath, leaving rugged mountains, the 

 height of which depends upon the amount of original uplift and 

 the subsequent denudation. Laccoliths in various stages of de- 

 nudation occur in different parts of the West. Fig. 128 shows 

 Little Sun Dance Hill in South Dakota, a small dome from which 

 the overarching strata have not been removed and the igneous 

 core has nowhere been exposed, yet there can be little doubt of 

 its presence. In the same region is Mato Tepee (also called the 



