6 
CHEMISTRY: CLARKE AND STEIGER 
whole earth crust. There come to mind the suggestions of George Darwin 
as to the possibihty of tidal waves in the earth crust as orogenic forces that 
would cause an equatorial N-S direction in the folds and a NE direction in 
the northern hemisphere; and those of Douville and Prinz who would ex- 
plain, one the prevailing E-W, the other the N-S directions of many of the 
mountain ranges of post-Proterozoic time by a former greater velocity of 
revolution (Suess). It would seem that these belts of Pre-Cambrian folds 
lend themselves still more readily to an explanation by one or the other of 
these factors than do the post-Proterozoic more local fold systems. Since 
the Pre-Cambrian folding both in America and Eurasia has a southerly com- 
ponent (thrust from SE and SW), a retardation of the revolution of the Earth 
and a resulting wandering of the crust towards the poles seems to be 
indicated. 
On the other hand, it must also be asked whether the world-wide folding 
of the Archean basement complex could not be explained by simply terres- 
trial forces. In this connection the result of close mapping of the Pre-Cam- 
brian folds, carried out in late years in Bohemia and Scandinavia is of great 
importance. It brings out closely compressed folds, whose strikes are tor- 
tuous and wavy curves and often subcircular and even angularly broken 
lines. This, it has been concluded, points to a tangential pressure, acting 
from all sides on an earth crust of fairly uniform composition (Uhlich), a pres- 
sure and a composition that could be found only in Pre-Cambrian or rather 
Archean time, and that means a uniform contraction of the entire earth crust 
such as could not be invoked for the post-Proterozoic mountain systems. If 
this view should supply a competent explanation for the world-wide Archean 
folding, it still leaves unaccounted for the presence of large systems of uni- 
form folding, which, as we have already seen suggests the view that in Pro- 
terozoic and even in Archeozoic time the crust was separated into masses 
that correspond in position if not in area and configuration to the continents 
of Paleozoic and more recent time. 
THE INORGANIC CONSTITUENTS OF LOBSTER SHELLS 
By Frank Wiggles worth Clarke and George Steiger 
U. S. Geological Survey, Washington 
Read before the Academy, November 18, 1918 
In the course of an extensive investigation relative to the inorganic con- 
stituents of marine invertebrates, by Clarke and Wheeler,^ it was found that 
among the distinctly magnesian organisms the proportion of magnesia was 
dependent upon the temperature of the water in which the animals live. 
The cold water animals contained much less magnesium carbonate than those 
