J.W. Powell’s Survey of the Rocky Mountain Region. 3857 
tinued to have its seat through a long cycle in and about the 
same locality. The propylite is succeed a rock answering 
to Richthofen’s description of hornblende andesite, which is 
usually overlaid by a rock rich in augite with triclinic feldspar, 
which may be termed augitic andesite. Still higher in the 
series are found immense masses of trachyte which, however, 
is frequently intercalated with dolerite. The variety of the 
trachytes is very great—so great indeed that were it not for 
the persistence of certain mineralogical as well as textural 
characteristics, which are universally accepted as being dis- 
tinctive of that group of rocks, one might feel strongly 
tempted to make numerous subdivisions of them. ex- 
tremes of the varieties of the trachytes might be represented 
ing of well-developed orthoclase feldspar imbedded in a fine 
paste highly charged with peroxide of iron, Between these 
Tn the lithological scale, propylite and hornblendic andesite are 
very nearly intermediate between the extremes of acid roc 
pe " 
toward the acid end of the scale, and on the other toward the 
Sense independently of each other, so. that they intercalate ; 
the acid becoming at one end more acid with the progress of 
the volcanic cycle, and the basic rocks becoming more basic. 
, series seems to pursue its own order and to be subject to 
its own law, so that being originally divergent, they become 
more and more widely separated in their lithological charac- 
Am. Jour. oe ae Vou. XV, No. 89.—May, 1878. 
