TRACHYTIC ROCKS. 
505 
pearance from basalt, for although rarely so heavy, dark-col¬ 
ored, or compact, it may present at times all these varieties 
of texture. Both these rocks are composed of triclinic feld¬ 
spar and augite with more or less olivine, magnetic or titan- 
iferous oxide of iron, and usually a little nepheline, leucite, 
and apatite; basalt usually contains considerably more oli¬ 
vine than melaphyre, but chemically they are closely allied, 
although the melaphyres usually contain more silica and 
alumina, with less oxides of iron, lime, and magnesia, than 
the basalts. The Rowley Hills in Staffordshire, commonly 
known as Rowley Ragstone, are melaphyre. 
Gree7isto7ie,—T\ii^ name has usually been extended to all 
granular mixtures, whether of hornblende and feldspar, or of 
augite and feldspar. The term diorite has been applied ex¬ 
clusively to compounds of hornblende and triclinic feldspar. 
Labrador-rocJc is a term used for a compound of labradorite 
or labrador-feldspar and hypersthene; when the hypersthene 
predominates it is sometimes known under the name of .fiy- 
persthene-7'ock. Gabhro and Diabase are rocks mainly com¬ 
posed of triclinic feldspars and diallage. All these rocks be¬ 
come sometimes very crystalline, and help to connect the 
volcanic with the plutonic formations, which will be treated 
ofin Chapter XXXI. 
Trachytic Rocks. —The name trachyte (from rpaxvg^ rough) 
was originally given to a coarse granular feldspathic rock 
which was rough and gritty to the touch. The term was 
subsequently made to include other rocks, such as clinkstone 
and obsidian, which have the same mineral composition, but 
to which, owing to their different texture, the word in its 
original meaning would not apply. The feldspars which oc¬ 
cur in Trachytic rocks are invariably those which contain the 
largest proportion of silica, or from 60 to 70 per cent, of that 
mineral. Through the base are usually disseminated crys¬ 
tals of glassy feldspar, mica, and sometimes hornblende. Al¬ 
though quartz is not a necessary ingredient in the composi¬ 
tion of this rock, it is very frequently present, and the quartz 
trachytes are very largely developed in many volcanic dis¬ 
tricts. In this respect the trachytes differ entirely from the 
members of the Basaltic family, and are more nearly allied to 
the granites. 
Obsidia7i, —Obsidian, Pitchstone, and Pearlstone are only 
different forms of a volcanic glass produced by the fusion of 
trachytic rocks. The distinction between them is caused by 
different rates of cooling from the melted state, as has been 
proved by experiment. Obsidian is of a black or ash-gray 
color, and though opaque in mass is transparent in thin edges. 
22 
