HARD ROCK PHOSPHATES OF FLORIDA 717 



country. The explanation offered is as follows : The Upper Oligocene forma- 

 tions and some later deposits, now found on the uplands bordering the hard 

 rock phosphate fields, formerly extended directly across the country that is 

 now the hard rock phosphate area. The disintegration of these formations 

 supplied the miscellaneous material of which the deposits are composed ; the 

 solution of the underlying Vicksburg limestone permitted irregular and inter- 

 rupted subsidence through the formation of sinks, accounting for the mixing 

 of these materials ; the phosphoric acid was supplied from these overlying de- 

 posits, particularly from the Upper Oligocene formations, which are known to 

 be very generally phosphatic throughout their entire extent over western, north- 

 ern, and central Florida ; the phosphate and flint boulders were formed chemi- 

 cally through the agency of percolating water ; the hard rock deposits are 

 localized because the material in which they accumulated is lacking in uni- 

 formity ; local variations, particularly the presence of clay lenses, and other 

 conditions which interfere with the free circulation of water favored the for- 

 mation of phosphate boulders ; the plate rock represents fragments of disinte- 

 grated boulders; the hard rock phosphate deposits are limited by reason of 

 their mode of origin to a particular stage of physiographic development, in 

 which phosphate-bearing formations have disintegrated overlying a limestone 

 substratum, thus forming conditions favorable for the downward movement of 

 rain water carrying the phosphoric acid which reaccumulated at a lower level. 



ALTERATION PROCESSES AND PRODUCTS WITHIN THE QRENVILLE 



LIMESTONE 



BY ALEXIS A. JULIEN 



(Abstract) 



The following views have been derived chiefly from study of the Grenville 

 formation during several visits to Hull, Argenteuil. Ripon, Grenville, North 

 Burgess, etcetera, Canada, and to the Adirondack regions. The phenomena to 

 which the Grenville limestones owe their special interest are found to be 

 simply explained, partly by the reactions involved during alteration of the dolo- 

 mite (for example, "eozoon" structure) and partly (graphite and apatite) by 

 probable derivation from alteration of early organic forms — vegetable and 

 animal. 



In the Grenville limestone proper the evidences of derivation from detrital 

 sediments (rather than chemical precipitation) consist of the observed repeti- 

 tions of detrital and calcareous deposits, the peculiar lamination, and the 

 abundant distribution of graphite and apatite. The chief agencies of altera- 

 tion have been first magnesian and then siliceous solutions, resulting in two 

 great processes. First, doloinitization ; after consideration of hypotheses, it 

 is concluded that this has been due to saturation by magnesian solutions de 

 scending from a laterite cover. The subsequent process, silicitication. was two- 

 fold. Its earlier action was effected by thermal waters, also derived from the 

 laterite. The chief product was pyroxene rock after dolomite, with release of 

 calcium carbonate in alternation ( lirst variety (a) of "eozoon" structure). 

 Equal objections are advanced against tbe hypotheses Of Dawson ami o( King 

 and Romney. Other forms produced by different conditions comprised phlogo- 

 XLIX— Bull. Geol. Soc. Am., VOL, 24, 1012 



