LACCOLITHIC GENESIS 
209 
In Utah the Iron Springs laccolith, according to Leith and 
Harder,^® gives a similar figure for the superincumbent load, 
Lindgren fully agrees in placing the original depth of the 
Tuertos intrusion at not more than 3000 feet. 
Relations of Laccoliths to Sills. In the original definition it is 
stated as axiomatic that a-laccolith is merely a thickened sheet 
or sill or the latter is an attenuated laccolith. The Ortiz lacco¬ 
lith seems clearly to display features and relationships showing 
that this definition it not only not necessarily correct but that the 
two phenomenon are formed under entirely distinct tectonic con¬ 
ditions. Although in the same intrusive body one part is thick 
and laccolithic and another part thin and sill-like the two are 
sharply contrasted when brought into juxtaposition, and plainly 
appear to have responded to rather diverse tectonics. As else¬ 
where indicated the main laccolithic bulging is regarded as due 
mainly to intrusion at a point of potential orographic stress in 
which the already arching strata permits the magma to localize; 
and also in another direction to the presence of profound faulting 
which tends to facilitate bowing of the rocks along a single line 
by allowing a rock-prism readily to swing loose as it were at one 
end and seemingly float upon the swelling tide. 
On the other hand the intrusion of the thin sheet, or sill, ap¬ 
pears to take place irrespective of orographic stress or faulting. 
In the Ortiz case the laccolithic mass abuts a fault-plane cutting 
a great thickness of coal-bearing shales. On the northwest flank 
of the mountains, at Madrid the principal coal camp, there are 
four thick coal-seams which together with about 350 feet of shales 
and sandstones lie between two great sills. Each of the latter 
is over 200 feet in thickness and extends a distance of several 
miles out from the main igneous body. The relationships of the 
sills to the coal-seams suggest that the intrusive sheets themselves 
have perhaps followed the paths of former coal-seams, possibly the 
most extensive of the series, as horizons along which they were 
able to insinuate themselves and advance most easily. Similar 
phenomena are not unknown elsewhere. In the Scottish coal¬ 
fields intrusives take the place of coal-seams for long distances 
15 Bull. U. S. Geol. Surv., No. 338, p. 47, 1908. 
16 Mineral Deposits, p. 647, 1913. 
17 Mem. Geol. Surv., South Staffordshire, p. 118. 
