28 The American Geologist. Jul y- 1905 
Ortiz are the Sandia mountains, a range of the basin type 
whose steep western face marks a fault scarp with a throw 
of over 4,000 feet. On the east the Sandias have a gentle 
slope and the beds dip gently east. The gentle easterly dip 
persists for many miles, and across the edges of these dip- 
ping beds a plain has been cut. The plain is not perfectly 
flat but has irregularities due to two causes. One of these 
is the Rio Grande and its tributary, Galistro creek, which 
have begun to dissect the plain ; the other the difference in 
hardness of the various rocks cut, the edges of hard beds 
standing up in cuesta-like scarps. The hardest beds in the 
region are igneous sheets, derived from the Ortiz mountains. 
The Ortiz laccolith was intruded after the strata were 
tilted to the east. Its cover has been largely removed by 
erosion, and the tops of the central and highest mountains 
(whose altitude is a little short of 9,000 feet) consist of the 
igneous core. Across the edges of the surrounding strata 
a plain has been partly built and partly cut, this plain slop- 
ing away from the laccolith on all sides. Because of its 
outward slope in all directions this form is here named a 
conoplain, and its slope is partly cut and partly built. This 
conoplain becomes continuous below with the general level 
of the region, at an altitude of about 5,800 feet. The cono- 
plain has been cut alike across the Cretacic beds and the 
igneous sheets, and upon its surface has been deposited 
alluvial material (the Santa Fe marl of Hayden). It is not 
to be understood that this plain is a smooth surface with 
the configuration of a cone; on the contrary the harder beds 
stand above the soft to the extent of upwards of a hundred 
feet. Rut a line drawn from the central mountains outwards 
in any direction will pass over a surface cut on the sur- 
rounding rocks and sloping upwards towards the mountains. 
It is confidently believed that such a form is the normal 
one in a mountainous arid region, differences of topographic 
age being marked by differences in slope. 
The difference in altitude between the mountains and 
the surrounding plateau (a difference of about 4,000 feet) is 
sufficient to produce a marked difference in precipitation. 
Vegetation is the measure of precipitation. The mountains 
catch the rain, and are in consequence forest-covered, with 
