PENNSYLVANIAN FORMATIONS IN THE RIO GRANDE 811 
feet thick overlaid by about twenty-five feet of shale and thin lime- 
stone above which occurs two feet of quartzite followed upward by 
fifty feet of compact bluish earthy limestone. It is at the contact 
of the shale and limestone formation with the conglomerate below 
that the “upper” or “surface” ore deposits occur at Kelly. 
These Sandia beds are well exposed on the east slope of Mount 
Socorro, and northward in the Limitar Mountains. They also appear 
on the east side of the river opposite Socorro and elsewhere. South- 
ward, as stated above, there is a marked decrease in the porportion of 
sand and clay beds accompanied by an increase in calcareous sedi- 
ments. ‘The shaley beds which constitute the lower part of the escarp- 
ment near Hermosa are evidently the equivalent of the Sandia beds 
as represented in Socorro County, but farther south the distinction 
between the upper (Madera) and lower (Sandia) divisions of the 
Magdalena is not recognizable. 
The thickness of the sediments referred to this division varies 
from 500 to 700 feet. 
Madera limestone.—Overlying the Sandia beds in Socorro and 
Bernalillo counties is a dark-blue limestone, for the most part in 
thick beds alternating with other thin shaley beds and blue shale. 
The limestone contains many fossils of the same type as those found 
in the Sandia beds below, but owing to the extreme hardness of the 
rock specimens are difficult to obtain. 
These limestones constitute the top of the ridge anor Kelly, 
where they have a thickness of about 300 to 500 feet, having been 
partly removed by erosion. On Mount Socorro they reach a thick- 
ness of about 700 feet, while good exposures of the beds appear also 
in the Limitar Mountains. The Madera beds constitute the great 
limestone plate along the back slope of the Sandia Mountains, and 
upon this limestone plateau is located the little Mexican-town of 
La Madera, from which the formation is named.? 
Herrick? has described the lower part of the formation in the 
Sandias as consisting of dark limestone, while the upper beds are of 
massive, gray siliceous lime with an intervening sandstone or con- 
1 C. L. Herrick, Jour. Geol., Vol. VIII, pp. 114, 115, 1900. 
2C. L. Herrick, Bull. Univ. of New Mex., Vol. I, p. 104, 1899. 
3 C. L. Herrick, Jour. Geol., Vol. VII, pp- I14, 115, 1900. 
