50 FIELD CoLUMBIAN MuseEumM—GEoLoey, Vou. III. 
water was very different in composition and possibly far more corrosive 
than at present, have protected themselves by secreting relatively 
insoluble dolomite skeletons? — 
From their composition and structure it is very evident that 
these objects are accretions and not rolled fragments of preexisting 
rock. Therefore they have no bearing upon questions relating to 
subsidence. From the continuity between the living covering of 
the nodules and the calcite of the interior, as well as from the detection 
under the microscope of organic structure in this calcite it becomes 
certain that the accretions are of organic growth. They are not, 
however, individual animals, for organisms of many kinds are inter- 
mingled in them. They owe their existence to a sequence of events 
substantially as follows: The surface of the bank was covered with 
a soft calcareous ooze upon which coralline organisms could get 
no foothold. Upon this ooze certain gastropods and other shells 
were able to live. Also it is possible that the shells of dead animals 
may be transported to the bank by the current of the Gulf Stream. 
The Challenger secured living starfish there and other forms of h e. 
Such gastropod, echinoid and other shells provided the firm anchor- 
age denied by the ooze for encrusting calcareous organisms of many 
kinds. These, growing generation over generation, have built up the 
nodules. If the growth of the nodules is more rapid than the 
deposition of the ooze, then they will eventually coalesce and form 
a surface from which a coral reef may grow upwards toward the 
surface. 
THE SPECIFIC GRAVITY OF CLAYSTONES 

When it was attempted to compare the specific gravities of the 
concretions herein described with the densities of other concretions, 
it was found that apparently such densities had never been deter- 
mined. Therefore after the specific gravities of the specimens 
strictly comparable with those under consideration had been secured, 
the work was continued by the determination and comparison of 
the densities of fifty-four claystones from eight localities. 
The specific gravities were obtained in the usual manner by 
weighing in water after immersion to complete saturation. Clay- 
stones are permeable to water and absorb it in large quantities, but, 
after the first few minutes, very slowly. A constant weight in water 
is seldom attained with less than twelve to twenty-four hours immer- 
sion. Frequently the weight is appreciably constant only after 
treatment for several days. 
