A COAL-MEASURE FOREST 239 



The writer has described the Rio Grande valley as a region of 

 early uplift — in effect an anticline, though the anticlinal axis is not 

 exposed to view — and accordingly the dip of the strata is prevailingly 

 to the west on the west side and to the east on the eastern side of the 

 valley. The granite core of this anticline is largely removed, but is 

 exposed in the bases of the Sandia, Manzana, San Andres, and Organ 

 mountain ranges on the east side of the valley, as well as in the core 

 of the great Sierra Ladrone uplift and the base of the Limitar Mount- 

 ain on the west. 



The interval between these remaining buttresses of the great 

 eroded mass is now, for the most part, filled with late strata composed 

 of Tertiary (Santa Fe marls) and Pleistocene deposits, mostly nearly 

 horizontal, except in proximity to the areas of late volcanic disturb- 

 ance. There are also perforating necks and cones of basalt, as well 

 as interstrata flows and dikes of the same material, but of an older 

 period. 



In the American Geologist of recent date'' the writer announced the 

 discovery of granite masses in the valley in the midst of the Tertiary 

 beds, and gave a section extending from the Rio Grande east of 

 Socorro through these granitic bosses. This section is so taken as 

 to pass through the region here under discussion, though it does not 

 cut the exact locality in which the fossil coal flora was found. 



These granite ribs (Fig. i) are two in number, and are separated 

 by an interval of perhaps half a mile, which is partly covered by the 

 Tertiary beds, but, wherever cut by arroyos or canyons, the lower or 

 Sandia formation of the Coal Measures is exposed in basin-like 

 relation upon the granite, forming a shallow syncline (really a modi- 

 fied monocline). The western rib or elongated boss of granite 

 rises not over 100 feet above the canyon level, and presents a sharp 

 slickensided fault to the west, with a course some twelve degrees 

 east of north where measured. In some places the Tertiary wholly 

 covers the boss, and to the north and south the granite passes out 

 of sight beneath this formation, the entire length being perhaps two 

 miles, as exposed. The eastern exposure presents the overlying but 

 unconformable Coal-Measure formation consisting of shales, silicious 

 shale, quartzite, and ferruginous conglomerate near the contact, 



I " Laws of Formation of New Mexico Mountain Ranges," May, 1904. 



