368 PETROLOGICAL ABSTRACTS AND REVIEWS 
necessarily to be regarded as formed by solutions bringing soda from a 
foreign source.) 
Harker finds some rhyolites “‘without very distinctive characters” 
but the association with trachytes settles, for him, the question of their 
reference to the alkaline branch. This doubt must have been caused 
by lime, which is an “anomaly” in an alkaline rhyolite. Certain 
widespread basic sills “have little that is indicative of alkaline affinities,”’ 
but they are associated with some porphyritic dolerites of more sodic 
character, and these are elsewhere associated with more “typical alkaline 
rocks,” so the akaline nature of the first mentioned sills is considered 
established. Harker does not seem to realize that his argument also 
connects “typical alkaline rocks” with others of calcic character in a 
manner to illustrate the fact of transition from alkaline to calcic magmas. 
He refers to “‘augite-andesites which taken by themselves must be 
assigned to the calcic division” but these ‘‘aberrant types” are regarded 
as products of a “subsidiary differentiation”? which is apparently able 
to transgress the laws of ordinary differentiation. 
The cases just cited, and others like them, seem to justify the sus- 
picion that perhaps the north British Tertiary province illustrates the 
existence of intermediate parent magmas whose derivates through differ- 
entiation naturally tend in one direction to exhibit alkaline affinities or 
even ‘‘typical”’ characters, and in the other the more calcic varieties. Why 
is this not a natural result from differentiation of intermediate magmas ? 
The closing section of Harker’s address, on ‘‘Petrogenesis and Sys- 
tematic Petrography”’ is a plea for a genetic classification of igneous 
rocks. As in such arguments generally the necessity for many classifica- 
tions of these objects of tremendously complex relations and history is 
ignored. The essential difference of the one systematic classification 
upon which our nomenclature must rest, the system of petrography, from 
other classifications cf petrology is also ignored. The best illustration 
of this limited view of the situation is given by Harker’s reference to the 
present writer’s remark that “only generalizations without known 
exceptions in experience can be applied to the construction of a system 
that may be called natural,” to which Harker seems to think his own 
attitude is antagonistic, namely, ‘‘I hold, on the contrary, that such a 
science as geology can be advanced only by the inductive method, which 
implies provisional hypotheses and successive approximations to the 
truth.” The first opinion was expressed with regard to petrographic 
system only. The chief desideratum in a stable, non-theoretical, 
petrographic system is not the facility of naming a rock (important as 
