oe 
Hague and Tddings—Rocks of the Great Basin. 455 
our observation, that- there are none which can be classed as 
trachytes,—using the term, of course, in the strict sense in 
which it is now employed by most petrographers and probably 
by all-who make use of the microscope in the determination of 
crystalline rocks. A trachyte is that variety of voleanic rock 
in which the predominating feldspar is orthoclase, but which is 
so low in silica as to be free from secretions of quartz if fully 
crystallized. This definition agrees with that of both Zirkel* 
and Rosenbusch,+ “a Tertiary or Post-Tertiary quartzless ortho- 
clase rock.” 
strongly had this idea intrenched itself in the mind, that those 
feldspars whose thin sections showed twinned lamella only at 
one end were thought possibly to be orthoclase containing 
lamelle: of a triclinic feldspar, as suggested by Zirkel.t 
he application of optical tests to the feldspars of these so- 
called trachytes and questionable rocks of the Great Basin 
leaves no doubt as to their true nature. Simple Carlsbad twins 
when cut so as to give symmetrical extinction angles or good 
cleavage, prove to be quite basic plagioclase ; indeed a scarcity 
characteristic of the smaller sized porphyritic feldspars, the 
striations of the larger ones being usually so well developed as 
to be noticed in the hand specimen upon careful search with a 
pocket lens. ee 
Chemistry fully confirms these optical determinations. This” 
is well shown in the analytical work of the late Dr. George W. 
Hawes upon the “trachyte” of Mt. Rose, Washoe, Nevada, the 
results of which are published in detail in Mr. Becker's § recent 
report, — 
Similiar results bave been obtained by us from the “tra- 
chyte” of the Wahsatch Range, from characteristic rock of 
Eureka, Nevada, and from the voleanoes of the Pacific Coast. | 
In these cases the feldspars were isolated by the Thoulet 
F, Zirkel, Mikroskopische Beschaffenheit, 1873, p. 290. F. Zirkel, Explora- 
the Fortieth Parallel, vol. vi, 1876. p. 6. a 
si Rosenbusch, rage sip 8G ee Array main 1877, vol. ii, p. 179. 
Lode, p. 67. ks ; : ue 
| Notes on the Voleanoes of California, Oregon and Washington Territory, this 
 Jour., Sept., 1883. 
