112 
LACCOLITHIC STRUCTURES 
mountain structures. Now the New Mexican laccoliths are in¬ 
timately associated with a gigantic horst, which however finds no 
distinctive expression in the present relief configuration of the 
district. 
The tectonics of the great Sierra del Oro horst is well displayed 
at its sou-thwestern extremity where it reaches sky at the foot of 
the Sandia uplift. As exposed in the deep Tijeras canyon and in 
Hell canyon east of Albuquerque, the essentials of the structure 
there revealed are best represented by cross-section diagram 
(figure 6). Notwithstanding the fact‘that its actual existence 
seems so well established by abundant observation among the 
Fig. 6. Cross-section of Sierra del Oro Horst, New Mexico. 
Great Basin ranges as a mechanical possibility the mountain horst 
remained largely a strictly hypothetical feature until the founda¬ 
tions of one were found exhumed in the depths of the Sandia gor¬ 
ges. It is manifest that the sustaining stresses inaugurated in pre- 
Cambrian, or early Paleozoic, times continue to remain to the 
present day orographic potentialities of the first order. As a 
tectonic curiosity, as it were the bisection of the desert range of 
Sandia and Manzano by the Tijeras faults was pointed out some 
years ago; but the full mechanical significance of the horst 
structure was not then fully appreciated.® 
The chief stratigraphic horizon of the laccolithic intrusions is 
the great plane of unconformity at the base of the Carbonic lime¬ 
stones which rest directly upon the folded and beveled pre-Cam¬ 
brian crystallines. The intrusives directly abut a fault-plane and 
the channel of magmatic supply doubtless also followed the^ same 
5 Proc. Iowa Acad. Sci., Vol. XII, p. 167, 1905. 
