116 
LACCOLITHIC STRUCTURES 
It is not, however, necessarily intruded in the latter since Carbonic 
limestones are detected at many points on the flanks of the 
mountain. The laccolithic body is further distinguished by being 
located outside of the great horst and on the down throw side of 
the fault. The channel of magmatic supply na doubt lies in the 
fault-plane. In cross-section the structural features are indicated 
in the subjoined cut (figure 7). 
I 
Compared with other laccolithic mountains which have been 
described a novel phase is the loosened end of the overlying prism 
of strata. The genetic significance of this feature in laccolith 
development is at once apparent. As faults go an unique situa¬ 
tion is that there is on the same side of the plane of displacement 
a down-throw below and an upthrow above. However these are 
really widely different in date. Both of these characteristics pre¬ 
sent mechanical potentialities which are not possible with the 
ordinary concept of laccolith. They fully meet) the objections so 
often urged against the blister form of intrusion. 
Transverse Section of the Tuertos. In all general respects the 
structure of the Tuertos laccolith is identical with that of San 
Ysidro. The stratigraphic horizon of the intrusion is the base 
of the Paleozoics, which, as already mentioned, is the bottom of 
the Mid Carbonic limestones that rest directly upon the tilted Pre- 
Cambrian slates. The several relationships of the rocks are 
graphically represented below (figure 8). 
A distinctive feature is the presence of limestone on the back 
of the eruptive body. These limestones are highly meta-mor- 
phosed and notable contact deposits of metallic ores are associated. 
