VOLCANIC OR ERUPTIVE ROCKS. 211 



these volcanoes. It evidently came up through great fissures in the 

 Cascade, Blue Mountains, and Coast Range, and poured out on the 

 surface, flooding the whole intervening country.* The Deccan lava- 

 field, described by the Indian geologists,! is 200,000 square miles in 

 extent, 2,000 to 6,000 feet thick, and entirely without detectable 

 volcanic cones from which the lava could have come. These exten- 

 sive fields are mostly of basalt. In Utah and Colorado, according to 

 King and Endlich,j; rhyolitic and trachytic lavas reach a thickness 

 of 7,000 feet. As a general rule, outpourings of basalt reach the great- 

 est extent, but each sheet is thin, as if the basalt had been super/used ; 

 while acidic lavas like trachyte and rhyolite are outpoured in very 

 thick, sometimes dome-like masses, as if they had been only semi- 

 fused. 



In basaltic lava-fields a remarkable step-like or terrace-like appear- 

 ance is observable. The country seems to rise in successive tables or 

 benches. From this has arisen the term trap, from the Swedish word 

 trappa, a stair. This configuration is due to the abrupt terminations 

 of the successive flows (Fig. 185). 



Intercalary Beds and Laccolites. — Holmes, in Hayden's Eeport for 

 1875,* describes Mount Hesperus, Colorado, as wholly composed of 

 stratified rocks (cretaceous), with intercalated beds of eruptives, as if 

 the lava had forced itself between the strata. Such intercalary sheets, 

 which have been often observed by others, probably pass by insensible 

 gradations into laccolites — a new form of occurrence to which atten- 

 tion was first drawn by Holmes, but which has been elaborately de- 

 scribed by Gilbert || as characteristic of the Henry Mountains, and 

 other groups in the Plateau region. In this case the liquid matter 

 seems to have come up through fissures as usual, but, instead of break- 

 ing through to the surface, has lifted the upper strata and accumulated 

 beneath in great dome-like masses which, in fact, constitute the bulk 

 of the mountains (Fig. 186). The strata-covered dome thus formed is 

 afterward eroded, and the igneous core or laccolite is exposed. 



According to Gilbert, whether lava accumulates between the strata 

 or outpours on the surface is merely a question of relative specific 

 gravity. If the lava is lighter than the strata, then the latter will sink 

 and the lava be outpoured. If, on the other hand, the surface strata 

 be lighter than the lava, then the lava floats it up and accumulates be- 



* American Journal of Science, vol. vii, pp. 167, 259. 



f American Journal of Science, vol. xix, p. 140, 1880. Manual of Indian Geology, 

 p. BOO et seq. 



\ King, Geology of the Fortieth Parallel, vol. i, p. 632. Endlich, Hayden's Report 

 for 1876, p. 112. 



* Hayden's Report for 1875, p. 271. 



I Gilbert, Geology of the Henry Mountains. 



