44 Field Columbian Museum — Geology, Vol. III. 



gradually toward the center by a thickening of the walls and a 

 shortening of the cells to approximately equidimensional forms. 



The calcite of the body of the nodule is continuous with that of 

 the encrusting forms. There are certain exceptions to this, however. 

 A nodule (Plate XXVI, Fig. 2, and Plate XXVII, Fig. 2) of a shape 

 suggesting an enclosed shell, and about ten centimeters in diameter, 

 when opened disclosed the very light cellular calcite to a thickness 

 of ten to fifteen millimeters, and enclosing an annular core of denser 

 cream-colored rock with a line of demarcation perfectly sharp. This 

 hard material has the shape of a curved loop three millimeters thick, 

 inside of which the cellular material again occurs. It appears to be 

 a section through the shell of a large gastropod. Other sections of 

 specimens display the same character. Nearly all the nodules which 

 were opened contained shells of Pholas, or of some allied form. Some 

 specimens of the boring mussels were found, as well. The Pholas- 

 like shells had been dead for some time, and were filled with calcite 

 sand, but the syphon tubes penetrated in every instance either quite 

 to the surface or to within one millimeter of it. Small gastropods 

 completely enclosed and the calcareous tubes of worms are of fre- 

 quent occurrence. 



Such are the nodules from the Challenger Bank. The twenty- 

 eight specimens collected October 13, 1905, from the neighboring 

 Argus Bank are of the same general character, but differ in some 

 respects. They are smaller, and much more irregular in outline, as 

 well as darker colored from the presence of the calcareous algae in larger 

 numbers. They came from a depth of from 28 to 30 fathoms. They 

 vary in diameter from three to eight centimeters, and in weight from 5 

 to 212 grams. Some of the nodules have the spheroidal form of those 

 from the Challenger Bank, but many have no regularity of shape what- 

 ever. Some of them are simply shapeless intergrowths of branching 

 coralline forms, and others appear to be encrustations upon flat shells 

 of various shapes and sizes. The variety in form and size of the speci- 

 mens from the Argus Bank is well demonstrated by the specimens 

 forming the bottom row of Plate XXV. 



A chemical analysis of the substance of a nodule seeming desirable, 

 the inner part of an average specimen of about eight centimeters 

 diameter from the Challenger Bank was taken for the purpose. After 

 about one centimeter had been removed from the outside by chipping, 

 the remaining portion was pulverized to pass a 40-mesh sieve and 

 quartered down to convenient bulk for analysis. The analysis by 

 the author gave the following result: 



