KINDS OF ROCKS. AQ5 
distribution of each, that makes the case like that of tra- 
chyte and felsyte. ‘The author has proposed to use the pre- 
fix meta for metamorphic kinds when a rock occurs both 
metamorphic and eruptive ; but this is not intended to indi- 
cate a distinction in Aind, but only to abbreviate the qualify- 
ing word metamorphic. 
- According to the principles above stated, a rock having 
oligoclase or albite as its feldspar constituent cannot rightly 
have the same name with one having either of the dasie 
feldspars, labradorite or anorthite, as an essential part, 
although these feldspars are all embraced under the decep- 
tive title of plagicclase (p. 275). Between anorthite and 
- oligoclase there 1s a difference of 20 per cent. in the silica, 
and the former is simply a lime feldspar; and the contrast 
is large also between labradorite and oligoclase. Again, for 
a like reason, as already explained (p. 411), a mica-bearing 
rock containing little or no hornblende cannot properly be 
classed with hornblendic rocks. 
3. It has been supposed that pre-Tertiary crystalline rocks 
differed so decisively from the Tertiary and more recent, 
that those of the two series should not bear the same name. 
But geology knows nothing of any epoch of sudden transi- 
tion in the mineral nature of eruptive rocks at the com- 
mencement of the Tertiary era; on the contrary, it shows 
that the kinds made before and after this epoch are alike 
in mineral constitution, and differ not always even in tex- 
ture, but only in the greater prevalence after the Tertiary of 
volcanic or subaerial ejected masses, and therefore of rocks 
of the texture this involves. The distinction of doleryte 
from diabase, with others similar, is of this chronological 
kind. Rocks, hke other objects in science, should evidently 
be named from what they are, and not from the age in 
which they may have been made. 
4, Since quartz is the most abundant of all the minerals 
of the globe, it is the least characteristic of the ingredients of 
compound rocks. Recent lithologists have made it, in sev- 
eral cases, distinguish only a section under a kind of rock. 
Thus, there are dioryte and quartz-dioryte, felsyte and 
quartz. fels yte, trachyte and quartz-trachyte. On the same 
principle there are syenyte and quartz-syenyte, as adopted 
beyond. 
5. The division of crystalline rocks into acidic and basic 
rocks is explained on p. 274. The acidic afford on analysis 
