vi THE ELEMENTS OF ECONOMIC GEOLOGY 
Deposits (1913); for the bearing of physical chemistry 
on the formation of mineral deposits to Dr. R. H. 
Rastall’s recent Physico-Chemical Geology, and for the 
processes of rock formation to The Principles of Petro- 
logy, by Dr. G. W. Tyrrell, a colleague to whom I am 
indebted for much help in the preparation of this work. 
Theories of ore formation still show great but not un- 
natural differences of opinion ; the conclusions stated in 
this book differ from those of some standard works by 
regarding more ores as due to the deposition, solution, 
and redeposition of sedimentary metallic grains, and by 
accepting fewer ores as due to direct igneous processes ; 
and also by regarding the source of most metals as an 
ore-zone beneath the ordinary igneous rocks of the crust 
and not the igneous rocks themselves. Twenty years 
ago I put forward explanations of some fields on those 
lines, which were opposed by most of the authorities ; 
but opinion has since changed, and the extension of these 
conclusions may be expected to some other fields. Some 
of the authorities whose opinions are not adopted may 
feel that their conclusions have been dismissed too briefly ; 
but the need for brevity has prevented the full statement 
of the case for either side. The apparent discourtesy of 
the dismissal of eminent opinion without due notice is 
the discomfortable fate of those who write brief text- 
books on wide subjects. “A tale has seven variations, 
and all cannot be told if time is short,” was the tactful 
remark with which my East African headman once 
explained away a misunderstanding ; and the infinite 
variety of subjects with which Economic Geology deals, 
and the ambiguity of much of its evidence, give equal 
variability to the solution of its problems. If the book 
had been twice as long some views would not have been 
rejected with the apparent dogmatism rendered necessary 
by the limitations of space. 
J W. G. 
GLASGOW 
November, 1027