50 Field Columbian Museum—Geology, Vol. 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 li 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. 



