DREDGING ON THE COASTS OF SHETLAND. 79 
bed which they once inhabited was raised and became dry land; that it 
remained in that state for a period sufficiently long not only to destroy the 
entire brood, but also to semifossilize the shells by exposure to the action of 
the open air; that subsequently the tract was submerged, and again formed 
the sea-bed; and that it is still sinking by slow and perhaps imperceptible 
degrees. In this way the shallow-water species, as Nassa incrassata, would 
be gradually habituated to greater and greater depths, like the North At- 
lantic Starfishes. 
Living Mollusca, procured from a depth of 75 fathoms and upwards, being 
placed in a shallow vessel of sea-water taken from the beach, did not seem 
to be in the slightest degree affected by the sudden change of bathymetrical 
conditions. With a solitary exception (that of Cithara haliaéti), all continued 
for a long time vigorous and active. They fed, crawled, leapt, and swam or 
floated immediately below the surface of the water in an inverted position. 
One pair of Marginella levis was actually engaged for several hours together 
in celebrating the rites of “alma Venus”! It was not necessary to renew 
often the supply of water in order to ensure this state of liveliness. I kept 
many of them, and observed their habits, for three days, when they were 
killed for the sake of their shells. 
I consider all marine beds, of comparatively recent formation, to be 
necessarily fossiliferous, assuming that the same causes which now exist were 
in operation during that period. Evenif these beds do not contain the shells 
of Mollusca, the remains of other animals, or certainly some microscopic 
organisms (such as Foraminifera and Diatoms), can be detected by a careful 
examination. I am aware that this opinion is opposed to that of some able 
geologists. Mr. Geikie says, in his recent treatise ‘On the Phenomena of 
the Glacial Drift of Scotland’ (p. 126), ‘«I believe the greater part of this 
drift, though it is unfossiliferous, to be of marine origin. Its occurrence on 
water-sheds or on the sides of mountains and hills far out of the reach 
of any stream seems sufficient evidence that in such cases fluviatile action 
must have been impossible. And in these situations the mounds of sand and 
gravel are exactly comparable with others which occur in lower parts of 
the country. It is difficult, therefore, to avoid regarding the whole as due 
to the operation of some one general agency. This agency was, in all like- 
lihood, the waters of the ocean.’ The non-existence of fluviatile action in 
the places above referred to, at the time when the so-called drift was de- 
posited, does not seem to me proved, taking into consideration the changes 
of level which may have since taken place. In certain inlets or arms of the 
sea, rivers flowing into them may have a sufficient velocity to sweep the 
middle of the channel, and prevent the deposit of any mud or sediment 
which would shelter certain animals. Indeed the continual action of the. 
eurrent might preclude the possibility of any animal living within the pro- 
hibited area ; and in that case the central bed of the channel might be partly 
covered with clear sand, devoid of any organized structure. An illustration 
_ of this phenomenon will be found in Dr. Wallich’s account of Hamilton’s 
Inlet, Labrador. Butsuch cases are rare, if not exceptional ; and not only are 
the limits of such lifeless areas very circumscribed, but the absence of marine 
organisms may be attributable to the destructive property of fresh water. 
* Many kinds of animals are known to exist and flourish in the most rapid 
tideways and even in whirlpools; and the water of the ocean everywhere 
teems with life. The dredge has never yet failed to bring up organic re- 
mains from every part of the sea-bed which has been explored, however un- 
promising it may appear to the naturalist. Even in the cleanest-looking 
