340 Proceedings of the Royal Pltysical Society. 
may probably be discovered. But on the British coasts 
instances of such terraces seem to be very rare. The author 
of these remarks was at one time led to suppose that hori- 
zontal surfaces might as frequently be produced under 
water as above it. On further investigation, however, he 
sees that he was mistaken. The soundings given in the 
charts do not indicate in any place a regular level terrace 
below low-water mark ; and he would refer to the experience 
and observation of the members of the Eoyal Physical 
Society whether they can point out any instances of level 
terraces formed under the water in places that are ajBPected 
by the tide. The inquiry possesses considerable interest for 
those who engage in geological speculations. If, on full 
examination, it shall be found that, while level terraces above 
high- water mark are in constant process of formation, there 
are very few, if any, now forming under low-water mark, 
we are naturally led to conclude that the ancient sea-mur- 
gins found along our coasts were not deposited, as some 
seem to imagine, at the bottom of the sea, but were thrown, 
up on the shore, in the same manner as those that we now 
see forming there. As a further proof that the terrace or level 
platform which we so frequently find on the sea-shore, some 
10 or 15 feet above ordinary high- water mark, has been 
thrown up by the advancing billow, and has not been formed, 
as some suppose, beneath the water, we may refer to the 
fact, that the shells found in these terraces do not lie in the 
position which they must have occupied in their native bed, 
but are heaped together in a confused mass ; while no limpet 
or shell of similar nature is found adhering to the rocks that 
rise, above the level of the terrace. 
Heiglit of Terrace in Relation to the Ordinary Level 
of the Sea. 
When the terrace is formed by the advancing wave, its 
elevation above ordinary flood-mark must depend on the 
force with which the billow is driven against the beach. 
On those parts of the western coast of Scotland, for instance, 
which are exposed to the full swell of the Atlantic, they 
will rise much higher than on our eastern snores, and any 
