40 THE PHYSICAL GEOGRAPHY OF THE SEA. 
minutest organism that the most poweriful microscope can detect 
among the impalpable particles of sea-dust. This effect of diur- 
nal rotation will be frequently alluded to in the pages of this work. 
46. In its course to the north, the Gulf Stream gradually trends 
more and more to the eastward, until it arrives off the Banks of 
Newfoundland, where its course becomes due east. These banks, 
it has been thought, deflect it from its proper course, and cause it 
to take this turn. Examination will prove, I think, that they are 
an effect, certainly not the cause. It is here that the frigid current 
already spoken of (§ 11), with its icebergs from the north, are met 
and melted by the warm waters of the Gulf. Of course the loads 
of earth, stones, and gravel brought down upon them are here de- 
posited. Captain Seoresby, far away in the north, counted five 
hundred icebergs setting out from the same vicinity upon this cold 
current for the south. Many of them, loaded with earth, have been 
seen aground on the Banks. This process of transferring deposits 
for these shoals has been going on for ages ; and, with time, seems 
altogether adequate to the effect described. 
The deep sea soundings that have been made by vessels of the 
navy (Plate XI.) tend to confirm this view as to the formation of 
these Banks. The greatest contrast in the bottom of the Atlantic 
is just to the south of these Banks. Nowhere in the open sea has 
the water been found to deepen so suddenly as here. Coming 
from the north, the bottom of the sea is shelving ; but suddenly, 
after passing these Banks, its depth increases by almost a precip- 
itous descent for many thousand feet, thus indicating that the 
debris which forms the Grand Banks comes from the north. 
47. From the Straits of Bemini the course of the Gulf Stream 
(Plate VI.) describes (as far as it can be traced over toward the 
British Islands which are in the midst of its waters) the arc of a 
great circle as nearly as maybe, only the thread or axis of the Gulf 
Stream does not generally go quite as far north as the great circle 
would. Such a course as this is the course that a .cannon ball, 
could it be shot from these straits to those islands, would take. 
If it were possible to see Ireland from Bemini, and to get a can- 
non that would reach that far, the person standing on Bemini and 
taking aim, intending to shoot at Ireland as a target, would, if the 
earth were at rest, sight along the plane of a great circle, for the 
path of the ball would be in such a plane. 
