W. P. Jenney—Notes on the Geology of Western Texas. 25 
The serew which I used for this instrument had a millimeter - 
thread, and its head being divided into a hundred parts, gave 
hundredths of a millimeter. 
In testing the instrument with the higher power, I felt grati- 
fied that measurements could actually be made to the hun- 
dredth part of a millimeter, and confirmed by repetition. 
To make the instrument more generally useful, the objective 
of the telescope is provided with a draw-tube which adds four 
inches to its length, and permits observations to be taken at as 
short a distance as three or four feet. In addition, it is pro- 
vided with a brass cap, which can be slipped over the mounting 
_of the objective, and which carries a supplementary centered 
achromatic lens, which permits the instrument to be approached 
to within a distance of six inches from the object. When thus 
arranged it enlarges the actual dimensions of objects about forty 
diameters, but even with this rather high power, the smooth- 
hess and steadiness of the micrometric movement seems in no 
degree impaire 
Columbia College, Nov. 4th, 1873. 
Art. VI —Notes on the Geology of Western Texas, near the thirty- 
second parallel; by WautTER P. JENNEY, E.M., Geologist 
Texas and Pacific Railroad Survey in West Texas. 
I. On the occurrence of aremarkable Lower Silurian Section in the 
Organ Mountains. 
WHILE examining the mountains about two miles north of 
EI Paso, Texas, which are a continuation of the Organ range 
southward from Solidad Pass, I was surprised to find a great 
development of the Paleozoic rocks, with the divisions between 
the strata of the different periods very distinctly marked, and 
each bed filled with an abundance of well-preserved specimens 
of its characteristic fossils, 
The strata dip at an angle of about 20° to the west, toward 
the valley of the Rio Grande; and the upturned edges of the 
ifferent beds form the eastern slope of the range, where, begin- 
hing at the foot hills at the base of the mountain, the section is 
as follows : 
A: A coarsely crystallized red-feldspar granite, which extends 
along the whole length of the eastern base of the range, and 
where exposed has a thickness of 100 feet or more. oe 
oy tay quartzite, resting unconformably on the granite: 
the lower portion of this stratum is hard and compact, and con- 
‘ans some chlorite slate. but it merges as one ascends into a 
