SCIENTIFIC SUMMARY. 
275 
Clare, blocks of limestone, which formed part of a bed two or three feet 
thick, have been wasted by the atmosphere, till all now left to represent 
them are mere cakes an inch or two thick. 
It is believed that this great atmospheric denudation has been going on 
ever since the carboniferous period ; for, excepting the depression during 
the glacial epoch, Mr. Jukes regards it as certain that the South of 
Ireland has been dry and high continuously from that time. 
Should this explanation be generally accepted, of course it must be 
applied to the rivers of the Weald, and many in the North of England ; 
while it will at the same time be coneeded that all the minor features of 
the districts are the result of a gradual action on the soluble rocks, like the 
melting of a jelly-fish in the sun. 
The remains of a large placental mammal have been detected in the 
upper greensand by Mr. Harry Seeley. 
Preservation of Fossil Shells. — It is well known that shells often consist of 
two distinct layers ; an outer one secreted by the margin of the investing 
membrane called the mantle, and an inner pearly layer secreted by the 
whole of the mantle. The pearly layer is composed of the form of car- 
bonate of lime crystallizing prismatically, called aragonite, while the outer 
layer is often the other, or rhombohedral form, called calcite. And it 
has for some time been observed empirically, that the shells of the Chalk 
have all lost their inner layers ; while in some other deposits, such as the 
Portland oolite, Cambridge greensand, &c., multitudes of the fossils have 
lost the whole of their shells, and exist only as moulds, while some 
other genera existing with them, and often nearly allied, always have 
the shell preserved. This has induced Mr. Sorby to make experiments 
on a large number of recent and fossil shells ; and as the result, he finds 
that when the shell is preserved it is in every case calcite. And that 
shells in the state of aragonite, such as Trigonia, always have a tendency 
to become crystalline by taking the form of calcite ; and if in this change 
the conditions are more favourable for crystallization in some other place 
rather than in situ the shell disappears altogether. 
MECHANICAL SCIENCE. 
The Middle Level Drain. — That necessity is the mother of invention 
is nowhere more frequently illustrated than in engineering practice, and 
the inundation of Marshland, in consequence of the undermining of the 
quicksand foundation of the great outfall sluice of the Middle Level drain, 
has caused the engineer to resort to several novel expedients ; one or two 
of which from their value and ingenuity deserve permanent record. The 
drainage waters of the Middle Level district are conveyed through Marsh- 
land to the Ouse by the Middle Level drain, which had at its termination 
an outfall sluice with three openings, each twenty feet wide, to permit the 
discharge of the drainage at low water, and to prevent the reflux of the 
tide. During March and May last, the bank at the wing-wall of the sluice 
was undermined, and gave way, admitting the tide to the drain, and. 
