588 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lipores that were dead ; this side evidently had recently been undermost. 

 They thus agree exactly in external appearance with the " pebbles " of 

 the Challenger. But while both Thomson and Murray believed them 

 to be composed of the same material as the Bermuda serpuline reefs, 

 that is, of aeolian limestone, careful examination has shown them to be 

 of an entirely different origin. When sawed into slices, the sections show 

 that the pebble is composed of successive, more or less regularly con- 

 centric layers of Lithothamnion, together with a few worm tubes, bry- 

 ozoans, and other incrusting organisms. The whole mass is more or less 

 porous, often honeycombed by the borings of mollusks (Lithophagus), and 

 the looser regions alternate with more compact layers. The mode of 

 growth has evidently been that of successive depositions of lime by nul- 

 lipores around some small central core, wliich may be very minute. This 

 is not, of course, in any sense of the word a pebble, but is a true nulli- 

 pore "head," in no way to be confounded with either aeolian pebbles or 

 the mechanical concretions of lime or siliceous materials so often dredged 

 in deep water. 



The spherical form of these heads is of considerable interest. Such 

 spherical nullipore masses occur very commonly in shoal water, either as 

 concretions about some core, or as independent stalked growths, which 

 eventually become detached and free to roll about. Such forms have 

 been taken in great numbers off Eastport, Me., in a few feet of water, 

 and also in many other localities where they are well within the sphere 

 of wave action, and probably owe their spherical form to the fact that 

 they are frequently rolled over and over. Such spherical masses have 

 also been previously dredged in deep waters, where their mode of growth 

 has been something of a problem. That some such process of wave ac- 

 tion is at work on the Challenger Bank seems probable from the spherical 

 form and worn appearance of the concretions, the latter condition being 

 made evident by the broken nullipore branches. Without being occa- 

 sionally overturned they would not attain a spherical form, but would 

 probably take on forms similar to those of porites, astraeans, or other 

 fixed massive corals. To account for the amount of disturbance of the 

 bottom necessary to explain such spherical concretions, we must turn to 

 a consideration of wave action ; neither tides nor currents being suffi- 

 ciently strong in this locality to afford an explanation. 



Statements as to the depth to which the action of storm waves may 

 extend are conflicting and unsatisfactory in the extreme. While it is 

 often claimed that one hundred feet marks the limit to which wave action 

 is important in the transference of material, I am assured by Professor 



