LACCOLITHIC STRUCTURES 
109 
NEW MEXICAN LACCOLITHIC STRUCTURES 
By Charles Keyes 
Geographic Characteristics. The Sierra del Oro, or Gold 
Mountains, constitute one of the oldest and most famous of Ameri- 
can mining districts. The title is that applied by the early Span¬ 
ish settlers of northern {New Mexico to certain conspicuous 
eminences rising abruptly out of the vast desert plain which en¬ 
compasses the southernmost extension of the Rocky Cordillera, 
and which is also one of those illimitable bolsons that characterize 
the high Mexican tableland and the Great Basin region of western 
United States. Apparently these mountains rest on the bottom 
of a broad valley having the last ranges of the Rockies on one 
rim and on the other the first of the Basin ranges. 
Viewed from a distance of a score of miles, from the Capitol 
dome at Santa Fe, through the dense gray haze of desert, the Gold 
Mountains appear as four groups of sharp peaks which rear their 
jagged heads above the surrounding plain like rocky isles above 
the surface of smooth summer seas; or lengthened and distorted 
by the desert mirage they seem through the dust fog as clusters 
of church spires of some great and populous city. In order from 
north to south the several groups of peaks are Los Cerrillos Hills, 
the Ortiz Mountains, the Tuertos Mountains, and the San Ysidro 
Mountain. Beyond are the great Sandia-Manzano ridges — the 
first of the so-called Basin ranges. 
Besides jutting abruptly out of the plain these four mountain 
groups constitute curious andesitic masses embraced in a field of 
sediments. That the eruptive bodies are not normal extrusives 
is clearly indicated by the fact that the Carbonic limestones and 
Cretacic shales are upturned ' around their flanks. They are 
laccoliths as typical as can be found anywhere on the face of the 
globe. 
