LACCOLITHIC STRUCTURES 
111 
they too would appear as erratically arranged as are the Henry 
Mountains of Utah, or the Judith Mountains of Montana. 
Tectonically the position of these mountains so near, yet quite 
apart from the last vestiges of the Rocky Cordillera, is frought 
with the utmost significance. To these curious relationships 
attention is particularly directed later. The same compressive 
stresses by which were corrugated the southern Rockies seem to 
find distinctive expression far beyond the visible geographic 
boundaries of the Cordillera. Analysis of the geologic structures 
associated with the formation of the Sierra del Oro indicate for 
it a composite orographic origin. 
That the several laccoliths under consideration should be reg¬ 
ularly arranged according to a definitely determined plan, instead 
of being promiscuously disposed, as is claimed for most mountains 
of this character, appears to have been predestined long prior to 
the date when the intrusion took place. The controlling cause is 
a pre-Cambrian consequence. Geological structures which were 
thus early initiated persist through the ages to become determining 
factors in the latest tectonics, and in the location of the intrusions. 
Tectonic Setting of Sierra del Oro. It is extremely doubtful 
whether laccolithic mountains ever have the erratic disposition 
usually accorded them. In the case of the New Mexican ex¬ 
amples of this class an adequate and satisfactory explanation for 
their regular alignment seems to be found in the older tectonics 
of the region. Two profound planes of fracture and displace¬ 
ment, parallel to each other and about four miles apart, appear to 
be late crustal adjustments along pre-Cambrian lines. 
The trough valley, or Grahen as the Germans term it, of which 
the Rhine valley is perhaps the best known example, is a 
structural phenomenon that) is familiar enough. Its origin is 
readily explained by assuming that it is formed by the dropping 
of a faulted block. The reverse of the drop-block, or raised block, 
is not so well known; and its genesis is even more difficult to 
account for upon any known principle of mechanics. Neverthe¬ 
less the phenomenon is one of not infrequent occurrence among 
the ranges of the Great Basin and the 'Mexican Tableland as well 
as elsewhere. On a small scale the phenomenon is commonly met 
with in mines where it is especially distinguished as a horse, or 
horst. The miner's title is applied by Suess in a larger sense to 
4Antlitz der Erde, I Bd., p. 167. 
