BYSMALITHS 795 
voir. The difference in the results must be occasioned either 
by differences in the direction and rate of the intrusion, or by 
variations in the resistance offered by the overlying rocks. A 
sudden vertical thrust may lead to the arching of the strata 
immediately above its point of application. Local weakness 
of the strata may cause their elevation in particular places, as 
remarked by Cross. Another cause undoubtedly lies in the initial 
arching of strata brought about by other dynamic forces tending 
to bend and dislocate rocks. Such movements may be accom- 
panied by extravasation of molten magmas which will follow 
planes of weakness in the dislocated rocks and will force them- 
selves most readily where the rocks offer least resistance to 
displacement. 
There is nothing in the petrographical characters of the rocks 
of intruded sheets and laccoliths in general to indicate any 
physical difference in the molten magmas from which each may 
have been formed. In both instances the magmas may have 
been equally liquid at the time of intrusion, or may have had 
similar compositions. 
As regards their shape, it may also be noted that in sheets 
the lateral dimensions are very great in comparison with their 
thickness, whereas in laccoliths the thickness is much nearer the 
lateral dimensions. 
Cross has shown that a certain amount of vertical displace- 
ment of the overlying strata may accompany their arching with- 
out changing the general character of the intruded body as in 
the laccolith of Mount Marcellina in West Elk Mountains, Colo- 
rado.* But, when vertical displacement with faulting is one of the 
chief characteristics of the intrusion, a distinction from normal 
laccolithic intrusion should be recognized. In the extreme this 
would result in the forcing upward of a more or less circular 
cone or cylinder of rock, which might be driven out at the sur- 
face of the earth, not necessarily in a coherent condition, or 
might be arrested at any stage of such extrusion and so might 
terminate ina dome of strata resembling the dome over a lacco- 
‘Loc. cit., pp. 182 and 236. 
