470 OLIVER 6 ATTKIN G DOWN, 
under the general name of propylites, which are well known in 
Hungary, Transylvania, Nevada, and from some South American 
localities. They present various aspects of the trachytic and 
trachyto-porphyritic structure, a different quantity and develop- 
ment of the elements of the first generation sometimes caus- 
ing the microlitic magma to predominate over the amorphous. 
An idea of the various aspects which these rocks present can be 
given by mentioning some from different localities. _Commenc- 
ing with the dacitic types, one may note the rock which occurs 
at various points in the mining district of Parral, in the state of 
Chihuahua. It is dark green to dull green in color, and contains 
scattered crystals of transparent feldspar, together with horn- 
blende that to the naked eye appears to be of a dark green 
color, and lamellz of dark green mica. The magma is of a 
character in part microfelsitic and in part microlitic, with dis- 
seminated particles of yellowish green hornblende, which is the 
mineral which gives to the rock its color. The crystals of 
hornblende are in part decomposed and do not always preserve 
their sections. This alteration, either central or peripheral, 
consists of a transformation to calcite, chlorite and sometimes 
to epidote. 
To judge by the free quartz which it sometimes contains, 
this rock bears some similarity to the felso-dacites of propylitic 
appearance, of Rosenbusch, and may correspond in part to the 
dacites as well as to the porphyrites of Fouqué and Lévy that 
are likewise analogous to some of the propylites described by 
Zirkel from the Virginia Range. In the same region these 
rocks sometimes have a lighter color owing to the abundance of 
disseminated feldspar crystals which give a more marked _por- 
phyritic appearance. There may also be observed with the 
naked eye and in very variable quantity, grains of pyrite dis- 
seminated in the paste. 
In the region of Guanacevi, Durango, altered andesites of 
green color form the rocks of the first eruption. With these are 
associated superposed andesitic tuffs and sometimes rhyolitic 
tuffs, likewise green, in beds of considerable thickness which 
