Hague and Iddings—Rocks of the Great Basin. 453 
exact coincidences at the first, fourth, sixth, seventh and ninth 
seconds, and the imperfect coincidences at the second, third ” 
and fifth. The rate of the fork as determined from it is 
3 50—22° 
The above results are submitted as the first obtained by the 
proposed method. I believe that, with certain improvements 
in some of the mechanical details, a considerably higher degree 
of accuracy may be attained, and the method be made as exact 
as the optical methods, with the additional advantage of per- 
maneuce. 
Art. LITI.—Notes ov the Voleanie Rocks of the Great Basin ; by 
ARNOLD HaGueE and JosepH P. Ippines, of the U. S. 
Geological Survey. 
published reports and maps of the exploration. In volume 
vi of these publications Professor Zirkel* presented the results: 
of his investigation and: determination of the crystalline rocks 
ased upon an examination under the microscope of many 
hundred thin sections. : : 
One result of this work has been to give a great impetus to — 
_ the study of microscopic petrography both in this country and 
-in England. Since the publication of Zirkel’s work micro- 
scopic petrography has made rapid strides, new methods have 
been introduced and many errors pointed out. Perhaps the 
* Microscopical Petrography, Washington, 1876. 
