Reviews—United States Geological Survey. 427 
various forms of calcium carbonate as well as an account of much new 
work regarding the conditions under which each form tends to occur. 
It is shown that under ordinary conditions there are only three 
definite modifications of the anhydrous salt, calcite, aragonite, and 
p-CaCO,, and one hydrated form, the hexahydrate. The various 
other forms, such as vaterite, conchite, and so forth, are proved to be 
varieties of either calcite or aragonite. Calcite is the stable form 
at all ordinary temperatures, but appears to undergo a reversible 
transformation at 970° to a closely similar modification a—Ca COs. 
Both aragonite and «—Ca CO, are metastable under ordinary conditions 
and tend to pass over to calcite. The rate of the irreversible 
transformation of aragonite to calcite increases with the temperature, 
but is very slow at ordinary temperatures. There is no experimental 
evidence for or against the view that aragonite is the modification 
stable below zero or under high pressures. The conditions under 
which the unstable forms precipitate are practically unknown, 
but the factors favouring their formation include a high degree of 
supersaturation, the presence of nuclei of isomorphous salts, the 
presence of sulphate in the solution, and the action of certain organic 
agencies. 
The paper also includes a critical examination of the various 
methods of distinguishing calcite and aragonite. The cobalt nitrate 
test, when used alone, is unreliable, as so many other minerals give 
the same reaction as aragonite and the effect of small quantities 
of impurities is so great. A similar conclusion is reached with regard 
to the ferrous sulphate test as well as in those advocated by 
Thugutt & Niederstadt, where the chief factor which influences the 
result seems to be the state of division of the material. The authors 
conclude that the chemical tests are, in general, unreliable and should 
be substituted by optical ones, which only fail when the material is 
sub-microscopic. 
Je 5 
ITV.—Unitep Srarrs Geotocicat Survey. 
“‘DULLETINS Nos. 620 and 621 of the United States Geological Survey 
constitute Parts I and II of Contributions to Economic Geology, 
1915. It is pointed out that there will be no volume corresponding 
to 1914, because commencing with the present volume the date is 
that of publication, and not the year in which the field-work 
concerned was done. ‘The papers in part i relate to minerals and 
oils, and in part ii to coal and oil. Mr. G. R. Mansfield describes the 
nitrate deposits in Southern Idaho and Eastern Oregon, which were 
first discovered in the spring of 1914 in or near the canyon of Sucker 
Creek, about 16 miles south-west of Homedale, Idaho; deposits have 
also been found at Jump Creek, about 10 miles further east. The 
nitre occurs, associated with sodium sulphate, in veins in rhyolitic 
Le but not in sufficient quantity to justify working on a commercial 
scale. 
Mr, Edward L. Jones, jun., briefly discusses the gold deposits 
near Quartzite, Arizona, close to the Colorado River Indian 
