359 I'he Oligocene of the Southekn United States 49 



between this rock and the overlying Tampa limestone, while 

 Conrad's description* in the same year is obscure in this respect. 

 Prof. Heilprinf speaks of a siliceous rock at Ballast Point, 

 containing quantities of Cerithium. But Dr. Dall;]; subsequently 

 found that the true siliceous rock does not contain Cerithium 

 {hillsboroensis) and thinks the stratum mentioned by Prof. Heil- 

 prin was probably a cherty layer of the Tampa limestone, over- 

 lying the Orthaulax bed. 



Bailey's infuso?'ial earth. Prof. J. W. Bailey, |1 in 1850, 

 found in the vicinity of Tampa Bay an infusorial layer which he 

 compared with similar beds in Virginia. Dr. Dall, in 1891, 

 observed this bed near Ballast Point, and found it to be a siliceous 

 marl, formed by local decomposition of the original Orthaulax 

 beds. In the samples examined microscopically, no infusorial 

 remains were found. § 



White Beach sand rock. The White Beach sand rock of 

 Little Sarasota Bay is a water worn limestone, covered with super- 

 ficial recent layers of sand. The fauna resembles that of the 

 Orthaulax bed, and also contains species of the Chipola marls.** 



The Tampa limesto?ie. The Tampa limestone which outcrops 

 along the shores of Tampa Bay, underlies the city of Tampa, and 

 extends to the Hillsboro and Manatee Rivers, was referred until 

 recently to the Vicksburg period. An outcrop of -this rock at 

 Fort Brooke, Tampa Bay, was described by Lieut. J. H. Allenft 

 in 1846, as a white limestone, typically hard, but with occasional 

 softer beds. Over wide areas it retains its general lithological 

 characfteristics. The country which it underlies is charadlerized 

 by sink holes and subterranean streams. Many lakes and ponds 



* Ibid., vol. 2. 



t Explorations West Coast, Trans. JVa^. Free Inst. Set'., vol. i, 1S87. 



tLoc. cit., p. 107. 



II Amer. Jour. Sci., 2nd ser., vol. 10, p. 282; also Microscop. Obs. Smith- 

 sonian Contr. Knowl., 1850, vol. 2, No. 8, p. 19. 



^Loc. cit., pp. 1 16-7. 



** Loc. cit., p. 114. 



ttSome Facts respeAing the Geol. of Tampa Bay, Amer. Jour. Sci., 

 2nd ser., vol. I, 1S46, pp. 38-42. 



