G. W. Tyrrell—Petrographic Nomenclatwre. 499 
maximum possible amount of salic constituents. In considering the 
norms another point of interest bearing on the question of classifica- 
tion is that the normative plagioclase remains practically of the 
same chemical composition throughout the series, the basaltic 
felspar being only slightly. more calcic than andesitic. While 
it is not urged that the average modal plagioclases of andesites 
and basalts are of nearly the same composition, the above fact 
certainly seems to tell against the distinction between andesite and 
basalt being based on the nature of the felspar only. 
Another interesting question concerns the nomenclature of the 
so-called hypabyssal rocks. Some authorities regard the minor 
intrusions or hypabyssal rocks as a classificatory division co-ordinate 
in importance with the plutonic and the volcanic. Others regard 
them merely as textural modifications of plutonic types consequent 
upon slightly different cooling histories. It is probably best to 
treat these rocks as textural modificatioas both of plutonic and. 
voleanic types, having chemical and mineral compositions 
corresponding with one or the other. This implies that, plutonic 
and voleanic rocks do not exactly correspond in chemical and 
mineral characters ; that is, the average andesite, for example, has 
not the same chemical and mineral composition as the average diorite, 
or the average basalt that of the average gabbro. This has been 
demonstrated by Daly in his studies of average igneous rock types.* 
The exact differences need not now be dwelt upon. It follows, 
however, that some hypabyssal rocks, porphyries for example, would 
best be described as andesite-porphyries, others as diorite-porphyries, 
and soon. It is probably the subconscious and implicit recognition 
of this fact that has led to many terms in hypabyssal rock nomen- 
clature gradually acquiring a textural significance, while stillretaining 
a specific meaning. Aplite and pegmatite are outstanding examples 
of this extension of meaning. They have well understood specific 
meanings ; when used alone they mean respectively granite-aplite 
and granite-pegmatite. But syenite-aplite, gabbro-pegmatite, and 
like terms, are equally well understood, in which the terms aplite and 
pegmatite have a textural meaning. It may well be questioned 
whether such an extension of meaning is not the inevitable end of 
all hypabyssal rock terms if these types be regarded, not as 
co-ordinate in classificatory value with plutonic and volcanic 
rocks, but merely as textural satellites of either group. The 
textural signification of aplite, pegmatite, and porphyry, may 
apply to the whole chemical and mineralogical range of igneous 
rocks. Aplites and pegmatites, however, are always satellitic to 
plutonic types; porphyries may be attached to either plutonic or 
volcanic types. Other hypabyssal terms, if used with textural 
significations, may, however, not apply to the whole range of 
igneous rocks. Felsitic, keratophyric, and orthophyric apply only 
toward the “ acid ”’ end of the igneous series ; bostonitic and tinguartic 
1 Igneous Rocks and their Origin, 1914, Table IT, p. 19. 
