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ELEMENTS OF GEOLOGY. 
called Neptune’s Cups {Spongia patera^ Hardw.), which grow 
in the seas of Sumatra; and if we could suppose a series of 
such gigantic sponges to be separated from each other, like 
trees in a forest, and the individuals of each successive gen¬ 
eration to grow on the exact spot where the parent sponge 
died and was enveloped in calcareous mud, so that they 
should become piled one above the other in a vertical col¬ 
umn, their growth keeping pace with the accumulation of 
the enveloping calcareous mud, a counterpart of the phe¬ 
nomena of the Horstead pot-stones might be obtained. 
Professor Wyville Thomson, describing the modern sound¬ 
ings in 1869 off the north coast of Scotland, speaks of the 
ooze or chalk mud brought from a depth of about 3000 feet, 
and states that at one haul they obtained 
forty specimens of vitreous sponges buried 
in the mud. He suggests that the Ven¬ 
triculites of the chalk were nearly allied 
to these sponges, and that when the silica 
of their spicules was removed, and was 
dissolved out of the calcareous matrix, it 
set into flint. 
Boulders and Groups of Pebbles in Chalk. 
—The occurrence here and there, in the 
white chalk of the south of England, of 
isolated pebbles of quartz and green schist 
has justly excited much wonder. It was 
at first supposed that they had been drop¬ 
ped from the roots of some floating tree, 
by which means stones are carried to some 
Small coral islands of the Pacific. 
laria radiata. 'D’Orb. But the discovery in 1857 of a group of 
White chalk. stones in the white chalk near Croydon, 
the largest of which was syenite and weighed about forty 
pounds, accompanied by pebbles and fine sand like that of a 
beach, has been shown by Mr. Godwin Austen to be inex¬ 
plicable except by the agency of floating ice. If we consid¬ 
er that icebergs now reach 40° north latitude in the Atlan¬ 
tic, and several degrees nearer the equator in the southern 
hemisphere, we can the more easily believe that even during 
the Cretaceous epoch, assuming that the climate was milder, 
fragments of coast ice may have floated occasionally as far 
as the south of England. 
Distinctness of Mineral Character in Contemporaneous Rocks 
of the Cretaceous Period. —But we must not imagine that be¬ 
cause pebbles are so rare in the white chalk of England and 
France there are no proofs of sand, shingle, and clay having 
