*S: p. Sharpies on Dr edgings from the Gulf Stream. 171 



ganic matter and amnionic carbonate and the further deposit of 

 iron, increasing the weight and thus diminishing the percent- 

 ages of the other components, 



Eock No. 3 is evidently composed in part of a rock like No. 

 2, ilerived from bone and in part of fragments of shells and 

 corals ; these fragments, being almost destitute of pliosphoric 

 acid, aid in reducing the percentage. _ The iron seems to be the 

 cementing material to a great extent in this case. 



No. 4 is more recent and consists almost entirely of decayed 

 shells and corals. The magnesia in Nos. 2 and 8 is evidently 

 derived from the sea water, it having replaced part of the lime. 



These specimens were selected from among a great number 

 as being typical of the different deposits met with in the dredg- 

 ing. These deposits may have a very near relationship to the 

 bone beds of South Carolina In the first place I think they 

 show that tendency of calcic phosphate is not toward con- 

 centration, but in every case we find the more recent the bone 

 or other material, the more abundant the phosphoric acid. 



Now in the case of the bone beds we have, leaving out the 

 silica, a substance of almost the same composition as bone with 

 the addition of a considerable amount of iron ; or, in other 

 words, a composition nearly approaching that of No. 2, with the 

 exception that there is less carbonate of lime. 



The much larger amount of sand is accounted for by the na- 

 ture of the surrounding coasts ; in the case of the Gulf, the 

 shores are mostly of coral, while in South Carolina they are 

 siliceous. We also have the bones of land and fresh water 

 animals mixed in with these deposits in such a good state of 

 preservation that they can be recognized, and occasionally we 

 find the imprint of shells from the beds below which may have 

 been exposed at some points. ^^ ^^ , 



There is another characteristic described by C. U. Shepard, 

 Jr.,* which applies equally well to the deposits from the Gul£ 

 He says the surfiice of the nodules is glazed over and pitted 

 with numerous small holes ; this would answer very well tor 

 a description of rock No. 2. . , 



Prof. C. U. Shepard's, Sr.f theory of the concentration ot 

 calcic phosphate in a mineral by the gradual removal of calcic 

 carbonate by solution in carbonic acid does not seem tenable in 

 view of the fact that calcic phosphate is also soluble m carbonic 

 acid. The large amount of alkaline carbonates that would also 

 be formed during such a process must also decompose to a con- 

 siderable extent the calcic phosphate. 



Cambridge, Dec. 1, 1870. 



* Tiiis Journal, H, vol. xlviL f This Journal, II, vol. ilvii, p. 338. 



