1050 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [G] 



iu Plate V, the temperature was lower (45° F.) at 30 fathoms tbau 

 even at the bottom, in 200 to 250 fathoms. There is, therefore, often a 

 stratum of colder water, in 20 to 40 fathoms, overlying the warmer Gulf 

 Stream water, between 50 and 100 fatlioms, in this region. This stratum 

 of cold water may be a lateral extension of the cold water of the in- 

 shore plateau, situated at similar depths. Perhaps the greater density 

 of the Gulf Stream water, due to evaporation, may so nearly balance 

 the increase in density due to lower temperature as to make this a phe- 

 nomenon of constant occurrence at these depths. 



It happened, not infrequently, that the surface temperature early iu 

 the morning, when we usually began dredging, was one or two degrees 

 lower than that at 5 fathoms, but during the middle of the day the sur- 

 face water was generally slightly warmer than that at 5 fathoms. 

 These changes are illustrated by some of the lines on Plates III and V. 



NATURE AND ORIGIN OF THE SEDIMENTS — OCCURRENCE OF FOSSIL- 

 IFEROUS MAGNESIAN LIMESTONE-NODULES. 



Lists of most of the stations occupied in 1881 and 1882 by the United 

 States Fish Commission steamer Fish Hawk have been given in a per- 

 vious article. In the lists the general character of the bo tom is indi- 

 cated, as well as the depth and temperature. 



A detailed description of the materials covering the bottom in this 

 region cannot be given at this time, but certain facts observed by us 

 are of sufficient geological interest to justify a brief notice. At several 

 localities, but especially at stations 1121, 1122, and 1124, in 234, 351, 

 and 640 fathoms, respectively, we dredged fragments and nodular 

 masses, or concretions, of a peculiar calcareous rock, evidently of deep- 

 sea origin, and doubtless formed at or near the places where it was ob- 

 tained. These specimens varied in size from a few inches in diameter 

 up to one irregular nodular or concretionary mass, taken at station 

 1124, in 640 fathoms, which was 29 inches loog, 14 broad, and 6 thick, 

 with all parts well rounded. This probablj' weighed 60 i^ounds or more. 

 The masses differ much in appearance, color, texture, and fineness of 

 grain, but they are all composed of grains of siliceous sand, often very 

 fine, cemented by more or less abundant calcareous matter. In some 

 the grains of sand are large enough to be easily seen by the naked eye, 

 and small quartz pebbles often occur in them, but in others the sand 

 grains are so fine that a microscopic examination is needed to distin- 

 guish them. These fine-grained varieties of the rock are often exceed- 

 ingly compact, heavy, hard, and tough, usually grayish or greenish in 

 color. They usually weather brown, from the presence of iron (probably 

 both as sulphide and carbonate). The sand consists mainly of rounded 

 grains of quartz, with some feldspar, mica, garnet, and magnetite. It is 

 like the loose sand dredged from the bottom in the same region. The 

 calcareous cementing material seems to have been derived mainly from 



