SCIENTIFIC SUMMARY. 
97 
mineral constitution as regards the chief constituents. When such dif- 
ference exists, rocks are different in kind, and need, for the purposes of 
geology, distinct names. If it does not exist, the distinction is only that 
of variety , unless (as in the case of trachyte and felsyte), the very wide 
extension of the rock under persistent characters makes a distinction of name 
important to geology. 
4. It follows from the preceding that differences in texture : as coarse, or 
fine, or aphanitic ; porphyritic, or non-porphyritic ; stony throughout, or 
haying unindividualised portions among the stony grains ; and differences 
in microscopic inclusions ; are no basis for a distinction of kind among rocks, 
but only of variety, and that porphyritic structure is of hardly more conse- 
quence than coarse or fine granular. 
5. No marked change in the constituents of the earth’s erupted material 
occurred after the close of the Cretaceous period, or just before the com- 
mencement of the Tertiary era ; and, hence, no ground exists for the dis- 
tinction of u older ’* and <l younger ” among eruptive rocks. The “ younger ’’ 
eruptive rocks are essentially like the “ older ” in chemical composition, and 
their chief mineral constituents ; and they differ, when at all, only in texture 
and some other points of as little importance — qualities that distinguish 
merely varieties, and which have proceeded from greater prevalence in these 
later times of sub-aerial eruptions. 
6. Since u plagioclase ” is not the name of a mineral species — several 
minerals of widely different compositions being embraced under it — it is a 
confounding of differences and resemblances to speak of it as a constituent 
of a rock. And since it now includes, through the defining of the feldspar 
microcline, a large part of potash feldspar, which had been supposed to be 
orthoclase, it has become almost synonymous with the term feldspar. The 
il simplicity ” its adoption has been supposed to give to lithological system 
would be greater if “ feldspar ” were substituted, and with its present range 
of constitution, the evil would be hardly less. 
7. Rocks differing mineralogically, and not chemically, like related korn- 
blendic and augitic rocks (the minerals hornblende and augite being dimor- 
phous) are rightly made distinct rocks, since the difference has depended, to 
a large extent, on wide-reaching geological operations or conditions, and is, 
therefore, of great geological significance. 
8. Since quartz is the most widely distributed and therefore the least 
distinctive of the minerals of rocks, it may rightly be regarded as of subor- 
dinate importance in the distinguishing of rocks, and hence not only such 
names as dioryte and quartz-dioryte, trachyte, and quartz-trachyte, &c., are 
acceptable, but also syenyte and quartz-syenyte. 
9. Biotite being closely like muscovite in composition, and not less 
common than it in granites, gneisses and mica schists, and being, moreover, 
unlike the mineral hornblende in chemical constitution and formula, the 
rocks in which biotite is a chief constituent cannot rightly be put in the same 
group with hornblende rocks ; or those in which hornblende is a chief con- 
stituent in a group of mica-bearing rocks. Consequently the name “ mica- 
dioryte,” for a rock containing no hornblende, and the name “ hornblende- 
granite ” for a rock containing no mica but hornblende instead, imply alike 
false relations. 
NEW SERIES, YOL. III. — NO. IX. H 
