270 The American Geologist, May, isqo 
these, after emerging from the mountains, quickly disappears 
in the loose gravels of the basin, probably the deposits of an 
old Tertiary lake bed. From each of the other ranges numer- 
ous streams and arroyas empty into the basin but the water 
all disappears shortly after reaching the plain. 
Near the western margin of the plain at the base of San 
Andreas range is an extensive salt marsh; and just to the 
south of this are the so-called white sands — a large deposit of 
gypsum. I am convinced that this is the last remnant of what 
was once a large lake now destroyed by desiccation. At pre- 
sent there is no surface drainage out of the basin and a very 
little brackish water collects in the lowest part of the old lake 
bed. It is completely isolated from the great drainage area 
of the territory and receives the rainfall of an area fully 4000 
square miles in extent. 
Conclusion. Condensing the above in a few words,I consider 
the history of the New Mexican drainage systems to first com- 
mence when the southern spurs of the Rocky mountains ap- 
peared above the sea. During their slow uplifting well estab- 
lished streams carved the existing mountain valleys flowing 
first into the sea and later into large bodies of fresh water. 
Grand subsequent uplifting has not appreciably affected the 
torrential mountain stream valleys. By the removal of bar- 
riers and probably in some cases by desiccation the lake beds 
became dryland and a young drainage began to establish itself. 
At this time the plateau uplift began and with it came the vast 
flows of basalt deluging the greater part of the lowlands of the 
territory. When this fiery deluge ceased the rejuvenated Rio 
Grande began anew, settling in many places in its ancient val- 
ley but in others encountering unexpected barriers. The river, 
then rejuvenated and superimposed has since been employed in 
cutting a deep canon. This stage has now ceased, the canon 
is beginning to lose its distinctive character, the side streams 
are repeating the example of the mother stream and burden 
ing it with an impossible load, which, being unable to carry, 
it is lying aside for some future time of leisure or power. 
NEW LAMELLIBRANCHIATA. 
By E. O. Ulrich, Newport, Ky. 
No. 1, Containing Descriptions of new species of Modiolopsis. 
This is the first of a series of papers on this class of fossil 
