Descriptive Arrangement of Volcanic Rocks. 31 
rieties being distinguished, according to the remaining pri 
— 5 means under the separate heels of— { 
. Tex nternal structure. 
9. Mision ‘disposition. 4. Natural division. 
_. There is, however, one previous distinction which it is in- 
cumbent to draw between the rocks of the trap family, and 
which is rather of a geological than mineralogical nature ; 
Viz, their division into the two classes of lithoidal or mass- 
ive, and fragmentary rocks ; —* as they are compo- 
sed of minerals intimately united b by the force of crystalline 
aggregation ; or merely of separate parcels of matter, =e 
herent, or enveloped in a cement, whether crystalline 
earthy, but evidently of later origin than the Fragmetited 
portion it encloses 
The second step is to arrange the lithoidal rocks into ge- 
nera, according to — broad ‘general characters of mineral 
constitution. _ Now it has been ascertained that all the rocks 
of this family, with me few exceptions, are —— com- 
posed of —— and augite in varying proportio -— 
plac 
et canis onThe Lets chelate the mate Senet 
occasionally replaced to a greater or less extent by mica; 
and this occurs only when the felspar ee peety in excess. 
The augite is either pyroxene or hornbl = the one seem- 
ing to replace the other in proportion to the abundance’ of 
felspar.. Titaniferous iron and. sphene are subordinate but 
very general ingredients in these rocks; the former is often 
in considerable quantity, and abounds most in the augitic 
species. Quartz occurs very rarely in crystals or grains; 
Eeinet, spinelle, sapphire, and other still rarer minerals can 
‘be reckoned as accidents. 
t is seen then tbat these rocks naturally group themselves 
into two pri | ers; viz—1. That in which felspar 
redominates exceotingly.- 2. That in which augite or the 
ous minerals are in excess ; or at least so abundant 
as to stamp their character on the rock. 'Thisi is, in fact, the 
division which has been generally made by the continental 
geologists, who have called the former order trachyte, the 
a 
t the dindés of mineral composition amongst the 
esd rocks of this family are so varied in nature, and ae 
