262 
GEOCHEMISTRY: CLARKE AND WHEELER 
and the greater number of suitable comparison stars which could be 
found for it. 
Another advantage of the selenium and photo-electric methods comes 
from the fact that the light of a body like a comet, nebula, or star clus- 
ter may be compared directly with a point source like a star. In addi- 
tion to photometric studies of diffuse objects, it is not too much to expect 
that we shall be able to get a very good measure of the general light of 
the sky, i.e., measures of the total light of the stars both in and out of 
the Milky Way. 
The photometry of lunar and solar eclipses also presents problems 
which can be attacked with success. The total light of the corona as 
well as its distribution will be easy to determine. Measures of the zodi- 
acal light can be made, and also any absorption of star light by these 
particles near the sun may be detected. This last would require very 
accurate observations, but would be feasible at a mountain station. 
The problems here presented are some of those for which we already 
have the means of attack, and which will lead to definite results. Be- 
sides what may be predicted in a way there is always the probability 
of something unexpected, and it is evident that only a beginning has 
been made in the use of the new methods. 
THE COMPOSITION OF BRACHIOPOD SHELLS 
By F. W. Clarke and W. C. Wheeler 
UNITED STATES GEOLOGICAL SURVEY. WASHINGTON 
Presented to the Academy, April 8, 1915 
The present communication^ is part of a much larger research rela- 
tive to the inorganic constituents of marine invertebrates. The object 
of the investigation is to determine, more definitely than has been done 
hitherto, just what substances are contributed by each group of organ- 
isms to the marine sediments, and therefore to the formation of marine 
limestones, with especial reference to their magnesian and phosphatic 
varieties. A few analyses of brachipod shells are already on record; and 
they show that they fall into two chemically distinct groups — one cal- 
careous, the other highly phosphatic. This conclusion is supported and 
emphasized by the new data obtained by us, which, however, bring out 
some minor peculiarities that seem not to have been previously observed. 
For our material we are indebted to the kindness of Dr. W. H. Dall, 
who selected typical specimens from among the duplicates in the col- 
lection of the U. S. National Museum. First in order come five brachio- 
