1880. ] Recent Literature. 285 
ture volcanets, so to speak, as may be seen on the flanks of Mt. 
Shasta. For example, “in the Uinkaret mountains, Major Powell 
has distinguished no less than *one hundred and eighteen erup- 
tive cones, and in the Henry mountains I have enumerated 
thirty-six individual laccolites. In one locality basic lava has one 
hundred and eighteen times risen to the surface by channels more 
‘or less distinct, instead of opening chambers for itself below. In 
the other locality porphyritic trachyte has thirty-six times built 
laccolites instead of rising to the surface.” In answer to the ques- 
tion, why in some cases igneous rocks form volcanoes and in 
others laccolites, it is stated that “when lavas forced upward from 
lower lying reservoirs reach the zone in which there is the least 
hydrostatic resistance to their accumulation, they cease to rise. 
If this zone is at the top of the earth’s crust they build volcanoes; 
if it is beneath, they build laccolites. Light lavas are more apt to 
produce volcanoes; heavy, laccolites. The porphyritic trachytes 
of the Plateau Province produced laccolites.” The process of 
formation is thus summarized: “The station of the laccolite 
being decided, the first step in its formation is the intrusion along 
a parting of strata, of a thin sheet of lava, which spreads until it 
has an area adequate, on the principle of the hydrostatic press, to 
the deformation of the covering strata. The spreading sheet 
always extends itself in the direction of least resistance, and if the 
resistances are equal on all sides, takes a circular form. So soon 
as the lava can uparch the strata it does so, and the sheet be- 
mountain structure is entirely due to Mr. Gilbert, although a 
careful reading shows that he recognized the fact that the Henry 
mountains are not the only examples of what he terms the 
‘laccolite.” Prof. Newberry, who examined the Sierra Abajo we o 
. 1859, was probably the first to recognize the:peculiar structure, o 
a : oe ; A RS, 5 ; 
