288 TRANS. OF THE ACAD. OF SCIENCE. 
western base, fragments of dark colored scoria were frequent 
ly observed. 
+ - 
Against the western declivity, the li strong 
ly upheaved, and where it is in immediate contact with the 
eruptive rocks, it is highly metamorphosed. I was much im 
were procured from these beds: Orthis testudinaria 
man), O. occidentalis (Hall), Rhynconella capax (Conrad), 
Rhynconella (a species of the Blue Limestone of Cincinnati) 
Streptelasma cornicula? (Hall), and columns of Crinoids. — 
In following the windings of the Pass, we first descended 
over thick deposits of loose and partially consolidated detr 
tus, after which nothing was encountered but granite 
limestone. The surface is generally hilly, and slopes towanis 
the Rio Grande, which is, here, of moderate depth, and flows 
over a roc . sa 
From this point to Dofia Ana, the course of the Expedition 
was nearly north, pursuing the valley of the Rio Grand 
which varies greatly in width, and is characterized by rw 
rich, alluvial soil, bearing a luxuriant vegetation. From 
east, the surface slopes towards the Rio Grande, and ons 
ern border is marked, for the first few miles above El 
y rt mountains of granite and porphyry, and afterwanis 
by precipitous bluffs, which form the eastern edge neh 
vated mesa, The soil is frequently encrusted : 
— a Acre salt and soda. io river has an a¥ 
width of about one hundred yards. e water is a 
ag and highly charged Fith fine yellow sediment. ™ 
El Paso Mountains exhibit the same general ap of the 
mineralogical composition as before. The limestone ° the 
miles not 
Lower Silurian system is seen strongly upheaved 
are exposed, in places, to the thickness of about tar 
dred. feet. Black ferruginous sand was observed. esti! 
retaceous Texas. e are 3 
siderably to augment the list by the addition of 2 we in the 
new and interesting forms, which have been place 
