
STRUCTURE OF ERUPTIVE ROCKS 2) Et 
midst of the lavas and tuffs of the Sidlaw Hills, the Ochil 
Hills, and other similar ranges in Central Scotland. From 
batholiths of all kinds proceed more or less numerous 
apophyses—sheets or sills, dykes, and veins—which penetrate 
the contiguous rocks often for considerable-distances. 
2. LACCOLITHS AND SILLS 
The name Laccolith has been given to certain remark- 
able masses of intrusive rock, which have been described by 
Mr G. K. Gilbert as occurring in the Henry Mountains of 
Southern Utah. The same type of structure has since been 
recognised in the Elk Mountains, and elsewhere in North 
America. As these laccoliths are of late Tertiary age, many 
of them are still in an excellent state of preservation, and the 
phenomena they present enable us to understand more 
readily the conditions under which certain of our own 
intrusive rock-masses may have come into existence. The 
general structure of a laccolith is illustrated in the accompany- 
ing diagram (see Fig. 66). It will be observed that the 

F1G. 66.—LACCOLITH. 
intrusive rock is lenticular in shape, that it sends out sheets, 
dykes, and veins into the contiguous strata, and is in connec- 
tion with a subjacent pipe-like feeder. According to Mr 
Gilbert, the molten rock has risen through this vertical pipe 
or fissure, but, being unable to burst across the superincumbent 
beds, has insinuated itself between the strata, lifted these up, 

