1881 .] 
263 
[Wadsworth. 
Again Mr. King remarks (I, p. 629) : “ Certain of the plagio- 
.clase trachytes , andesites , and propylites hear a striking resem- 
blance in the relation of their secreted minerals to the ground- 
mass bv which the resulting porphyries are puzzlingly similar” 
Of his observations he again says : “ But as since the period 
of the.se observations, quartz-propylite and dacite have been sep- 
arated, a doubt is thrown over the reading of that locality ” (I, 
pp. 683, 684). 
Mr. Emmons says of a “ rough gray sanidin-trachyte “ Both 
in habit and mineralogical constitution, this rock shows close 
affinity to propylite, and would be so classed, but that the ortho- 
clase predominates over the triclinic feldspar ” (II, p. 580). Zir- 
kel states that the same “ rock exhibits a considerable measure of 
similarity to propylite, and it would be so classed if orthoclase 
did not unquestionably predominate” (VI, p. 153). King also 
says of the same : “ The groundmass resembles that of propylite. 
* * * But for the predominance of sanidin over plagioclase, the 
rock, from the peculiar disposition of the hornblende, would be 
closely related to the propylite” (I, pp. 598, 599). 
The studies of the writer upon the propylites of the Fortieth 
Parallel Survey, as well as upon the collections made about Silver 
Mountain, in the Washoe District, and elsewhere, convinced him 
that the true propylite is an altered andesite standing in similar 
relations to it that melaphyr and diabase do to the basalts. Ko 
definite line can be drawn between the propylites an q andesites 
as the alteration products are the same, the difference being sim- 
ply in the degree of alteration. 
The hornblendic dust ; the color of the rock, of its groundmass, 
and of the hornblende ; the microscopical epidote ; the absence 
of a glass-bearing groundmass ; the rarity of augite, the struc- 
ture of the hornblende, etc., are in my opinion the result of alter- 
ation. Professor Zirkel states the reverse regarding part of these 
characters, yet he described rocks as andesites that cannot be dis- 
tinguished by me either macroscopically or microscopically from 
others belonging to the same district that he described as propy- 
lites. In the well marked specimens there is no difficulty in class- 
ing in the field, or by a mere examination of a hand specimen 
with the unaided eye, a rock as a decomposed or altered andesite 
