HEAT. 



.301 



space that may offer a way of escape ; and it may thus put layers of its own 

 material into the stratified series. 



(6) The lava of a fissure is always forced along by pressure from below ; 

 and if the fissure fails to reach the surface, the ascending stream may open 

 a space for itself by lifting the overlying beds, and accumulate in great 

 masses in the chamber so made. 



An intercalated mass of igneous rock formed in the latter way is called, 

 by G. K. Gilbert, a laccolite, from the Greek for lake, because it is a lake-like 

 expansion of a stream. (As the termination ite is that used for a mineral or 

 rock, the form laccolith, like that of monolith, is to be preferred.) The 

 thickness depends somewhat on the fusibility of the rock, the more fusible 

 kiuds making extended masses or sheets, and the less fusible producing 

 thicker and more bulging forms. 



The Henry Mountains in southern Utah are of laccolithic origin, and 

 they are those to which the term was 

 first applied by Gilbert. The following 

 figures are from his memoir (1877). 

 The greatest thickness of the strata 

 bulged upward by the lifting lava, in 

 the manner illustrated in Fig. 272, was 

 about 10,000 feet ; and the height of the 

 laccolithic dome in some cases is over 

 3000 feet. Fig. 273 represents an actual 

 laccolith, called Jukes Butte, completely 



stripped of its inclosing strata and deeply gorged by denudation. The rock 

 is andesyte, a rock less fusible than basalt ; and the breadth of the mass is 

 consequently only three to seven times greater than the height. 



273. 



272. 



Ideal section of a laccolith. Gilbert. 





Jukes Butte, a denuded laccolith, as seen from the northwest. Gilbert. 



From the laccolith rise dikes of andesyte. The sandstone adjoining is 

 usually more or less altered by the heat to a depth of a foot or more. The 

 chamber occupied by the laccolith was in all cases made along a shaly layer 

 in the formation where the cohesion was least. They occur at different 

 levels in the strata, and the one lowest in geological position is 4500 feet 

 below the level of the highest ; the former is between Carboniferous beds, 



