1881.] Geology and Paleontology. 831 
best seen in the subconical tubercles of the premolars, particularly 
that of the heel of the fourth inferior premolar. In the other 
three species this heel is more of a crest, and is connected with 
the principal cusp by a low ridge. The four species may be 
characterized as follows: 
_, 4+ Cusps of last premolars conical in both jaws. ; 
Size medium. ast lower molar disproportionately small ; cusps low, two anterior 
inner distinct; true molars, .018............+ M. turgidus. 
aa. Cusps of last premolars compressed in both jaws. 
Least. Second and third lower true molars subequal ; cusps, especially the internal, 
elevated; anterior inner confluent into an edge; true molars, .013 
M. angustus. 
Medium. Last inferior molar larger than penultimate; true molars, .014; p. m. III 
7. sectorius. 
] 
Largest, Cusps of inferior molars obtuse; p. m. II 008, its heel short and smal 
M. mandibularis. 
Of MW. turgidus there are two specimens; of JZ. sectortus three, 
and of WZ. angustus and M. mandibularis one each. 
GEoLocy oF THE Lake VaLiey Mininc Districr.—This dis- 
trict lies in the Eastern foothills of the Mimbres mountains, New 
Mexico, at the western border of a plain which extends from the 
Rio Grande. Several cuttings of the Atchison, Topeka and 
Santa Fé R. R. disclose the formation of the surface of the plain, 
while the shafts at the mines, and various monoclinal hills exhibit 
the general structure of the country. 
€ more elevated hills are dikes of trachyte-porphyry, and 
the directions of their axes are various. It partly decomposes into a 
reddish soil, and partly breaks down into gravel-like fragments. 
The lowest sedimentary rock I have seen in place is a quartzite, 
frequently not very hard, and often perforated by irregular cavi- 
€s. This material forms low hills, and is overlaid by one or two 
hundred feet of a fine carbonaceous shale from which most of the 
valleys are eroded. I was unable to determine the age of either 
this bed or the quartzite. Overlying the shale are from 150 to 
200 feet of more or less siliceous limestone, the upper part of 
Which is very fossiliferous. Professor White finds the fossils to 
demonstrate the age of this formation to be the Middle Carbon- 
Herous ; see July, 1881, NATURALIST. 
€ only formation found covering the limestone is drift. In 
the foothills this is composed of worn fragments of limestone and 
trachyte ; on the plain it mostly consists of fragments of basalt, 
with some trachyte, which are more or less coated with lime. 
_The rich silver deposit which is now attracting attention to this 
district, is found in the limestone, which forms low monoclinal 
hills between the higher trachyte ridges and the still lower hills 
S.S.W. from 12° to 30°. The silver-bearing rock form numerous 
veins which traverse the limestone from N.W. by W. to S.E. by 
» Or at right angles to the dips. The veins are of various 
