278 



THREE CRUISES OF THE " BLAKE. 



ill the Gulf of Mexico, and during my first dredging trip I was 

 struck with the change of the Hght yellowish-gray ooze of the 

 trough of the Gulf Stream into the darker shades as we ap- 

 proached the coast of Cuba. 



While running the line parallel to the coast from off Charles- 

 ton to Cape Hatteras, we came tw^ce upon localities where 

 the sounding-cup brought up nothing but clean globigerinse, 



the bottom consisting entirely 

 of the modern greensand ^ to 

 which Bailey and Pourtales had 

 already called attention as form- 

 ing off shore on the Atlantic coast 

 of the United States. From the 

 examination of a large number 

 of bottom specimens, Pourtales 

 succeeded in determininof the 

 position of a belt off the coast of 

 Georgia and of South Carolina 

 where the modern grreensand was 

 well developed. The patches of 

 greensand occurred in depths 

 varying from fifty to a hundred fathoms on the boundary line 

 between the siHceous sand and calcareous bottoms on the inner 

 edge of the Gulf Stream. Now and then this modern greensand 

 (Fig. 190) is found also in deeper water under the Stream itself. 

 Pourtales gives the following description of the formation of 

 our modern greensand : — 



Fig. 190. — Greensand. 142 fathoms. 

 (Blake Station 314. ) L5. 



" Ehrenberg made, as is well known, the interesting discovery that 

 the so-called greensand or glauconite consists of the casts of foramini- 

 fera.^ That this process is still going on at the bottom of the ocean, 



1 Murray says, that fifty per cent of 

 carbonate of lime, mainly made up of 

 pelagic and other foraminifera, and forty 

 per cent of greensand residue, enter into 

 the composition of one of the samples of 

 modern greensand he examined. The 

 remainder consists of siliceous organisms 

 and argillaceous and green amorphous 

 matter. 



^ Bailey confirmed the observations of 

 Ehrenberg, on the greensand of the Zeu- 

 glodon limestone of Alabama, and detected 

 the same structure in some of the creta- 

 ceous deposits of Xew Jersey and Texas, 

 and in the eocene of South and North 

 Carolina. 



