Geology. 299 



scalar notation being used throughout. Tract No. 6 constitutes 

 a short course in the special field with which it deals. It owes its 

 origin to a series of lectures given to the Mathematical Research 

 Class during the Spring Term of 1915. h. s. u. 



II. Geology. 



1. The Pebble Phosphates of Florida ; by E. H. Sellards. 

 Florida State Geol. Surv., 7th Ann. Rep. 1915 ; pp. 29-116, 52 

 figures. — The importance of the phosphate deposits of Florida 

 has long been realized and some knowledge of the geology of the 

 hard-rock phosphates has been available but, until the appearance 

 in September, 1915, of Dr. Sellards' paper, very little had been 

 known of the commercially more important pebble-phosphates. 



To give a clear conception of the origin of the pebble-phos- 

 phates Dr. Sellards describes the geology of Southern Florida 

 with particular emphasis on the late geologic history of the 

 region. The Tertiary stratigraphy is discussed in detail and the 

 fossils found are illustrated in a number of well-prepared plates. 

 It is pointed out that the hard-rock phosphate occurs chiefly as 

 pebble in marl of the Alum Bluff formation of late Oligocene age. 

 Its deposition was followed by an emergence and a period of 

 erosion. Another submergence in late Miocene or probably early 

 Pliocene allowed the sea to advance across the irregularly eroded 

 surface of phosphatic marl and there was formed first of all a 

 basal conglomerate, the pebbles of which are chiefly phosphate. 

 These deposits form part of the Bone Valley formation and the 

 basal conglomerate constitutes in places the workable phosphate 

 deposits. 



It is pointed out that the pebble-phosphate deposits vary in 

 thickness and continuity and the explanation is offered that this 

 is due to the irregularity of the erosion surface upon which the 

 pebbles were deposited and to the constantly changing depth of 

 water and force of wave action of the advancing sea. The varia- 

 tion in phosphate content of the deposits may be due to irregular 

 sorting action of water, to varying phosphate content of the 

 supply pebble, and to a secondary enrichment of the deposits, 

 more complete in some places than in others. 



The origin of the deposits as outlined by Dr. Sellards appears 

 to explain very satisfactorily the many perplexing problems that 

 these deposits present. The variation in continuity, thickness 

 and phosphate content may readity be understood by the hypoth- 

 esis advanced and should be of help in the mining of the 

 deposits, although no criteria are advanced which would help 

 exploration for more phosphate. The recognition of secondary 

 enrichment in the phosphate deposits is an interesting contribu- 

 tion. The value of this contribution might be enhanced if it 

 could be determined what controls the degree and location of the 

 secondary enrichment. a. m. batemak. 



