LACCOLITHIC STRUCTURES 
115 
line of rupture. In this regard the intrusives are strictly in- 
terformational in nature, like those recently described by Messrs. 
Weed and Pirsson ® in the Little Rocky Mountains of Montana; 
and by Prof. J. A. Jagger ^ in the Deadwood Gulch district of the 
Black Hills, although the first mentioned instance is more likely 
batholithic rather than laccolithic in character. However, the 
interformational nature of the laccolithic intrusions probably has 
no especial genetic significance, and the plane of separation of the 
strata is doubtless due mainly to the great ease with which parting 
takes place in some parts of rock-masses than in others and the 
resistance to ready fracture elsewhere. 
Ground-plan of the Laccolithic Groups. Unlike the laccolithic 
members of the Henry Mountains, the Judith Mountains, or the 
West Elk Mountains, in which no orderly arrangement or re¬ 
lationship is yet recorded the cuneiform intrusive masses of New 
Mexico all seem to align themselves along a straight course, as 
shown in the annexed diagram (plate viii). In this respect the 
faulting appears to be an essential condition of the intrusion and 
in point of time preceded it. From this straight thick edge of 
the laccolithic wedge the intrusive mass becomes antenuated in all 
directions. Instead of the laccolith being lens-shaped it is really a 
half-lens. 
In view of the circumstance that the ground-plan of laccoliths 
has been invariably assumed to be a circle, with a consequent 
erroneous perspective of position, these relationships are badly in 
need of strict review with conceptions in mind other than the 
original one. Reconstructed on a cuneiform basis instead of the 
blister hypothesis even the Henry Mountains appear to adjust 
themselves along regular lines which are parallel to the dominant 
tectonic features of the region — the great “Water-pocket” flexure. 
Deprived of their hypothetical extensions in circles around the high- 
peak points the Judith Mountains also assume definite tectonic 
relationships. 
Structural Features of San Ysidro. The southernmost mem¬ 
ber of the Sierra del Oro is best known as San Ysidro, but latterly 
is sometimes called South Mountain. A noteworthy fact con¬ 
cerning this intrusive mass is that it seems to be completely sur¬ 
rounded on the ground surface by Cretacic sandstones and shales. 
6 Journal of Geology, Vol. IV, p. 402, 1896. 
7 Annals Nexy York Acad. Sci., Vol. XII, p. 212, 1899. 
