NOTES AND QUERIES. 
176 
tooth is smaller ; its length on the giinding-surface eight and a half inches, by 
three inches wide in the centre, tapering oli' to one inch at each end ; it lias nine 
layers of dentine. — It may be useful to notice here that all the economic clays, 
viz., those for making bricks and tiles, found on the Red Marls, and also the beds 
of fine sand and gravel, are all of comparatively recent formation ; the true Red 
Mivrl — a mixture of clay and sand, — whether aliove the Kenper Sandstone or 
below it, cannot be made into bricks or tiles, and the tine clays used have been 
formed by the washing out of the Red Marl, the sand having been deposited in 
one place; and the clay more finely comminuted, and hence of less specific gravity, 
laid doivn in another. They are thus found in most Triassic districts as basins of 
clay of unequal depth and extent, and as beds of sand ; it is in these beds and 
basins, and in the gi'avel-pits, that the young geologist may expect to find 
mammalian remains. The geological maps of the Ordnance Survey, although 
generally very accurate in the boundaries of foimations, at least wherever I have 
examined them, are still defective in one particular and calculated to mislead. 
Miles of surface are laid down as Red jMarl, Lias, &c., where there are really drift- 
clays, sands, and gravels, of recent age, and this in places where the drift is 
upwards of 100 feet deep. What is wanted is a Map of the Drift. There is no 
doubt that, under this comprehensive term, a number of deposits of different ages 
are all confusedly gi-ouped together ; but such a map would at least greatly 
facilitate the search for Mammalian remains. — J. Plant, Leicester." — On the 
hirge diagi-am-map just executed under my direction, the coui-se and extent of the 
Great-Northern Drift is laid do\vn as far as our present knowledge extends. 
Additional information is still, however, very desirable. — En. Geologist. 
Sand-pipes near Swainstone, Isle or Wight. — " Dear Sir, — In accord- 
ance with the ^nsh of Mr. Prestwich, I beg to make known to your readers an 
interesting section of half a mile in length in this island, lately exposed by a 
cutting for a road near Swainstone, the seat of Sii" John Simson, Bart. Its 
interest chiefly arises fi'om its bearing on the date of the formation of swallow- 
holes,* in conjunction with the period of the upheaval of the vertical chalk-strata 
which are well known to form a belt through the Isle of Wight. At the spot in 
<luestion the chalk-strata are not quite vertical ; then- slight dip being to the 
north. The cutting divides the Plastic Clay (Woolwich beds) obliquely at its 
eastern commencement, and extends along the northern escaiijment of the central 
belt of chalk-hills. The uppermost beds of the chalk are divided at fust obliquely 
to their line of stratification ; but at the western end of the cutting these beds are 
cut through almost exactly parallel to their line of inclination. This cuttuig is of 
the width of an ordinary road, and presents very numerous sections of swallow- 
holes, nearly all of which have a southerly dip, i.e. across the line of stratification 
of the chalk. The inclination of the swallow-holes to the south is rather less 
than the dip of the chalk to the north ; hence the inference I draw is, that the 
swallow-holes were formed at a period subsequent to the first upheaval of our chalk- 
belt ; but at a period prior to that when the present verticality of the chalk was 
attained — a point to which I submit some interest may be fairly attached. The 
swallow-holes otherwise present features little difl'ering from those ordinarily 
met with. In no instance can I find flints which can be considered as water- 
woni ; for the most part they are fractured, and some are of very large size. In 
most of the swallow-holes flints may be seen hning, as it were, the circumference ; 
while the centre is composed of clayey gravel. In those nearest the eastern extre- 
mity of the section clay predomuiates, having a great similarity to the Plastic 
Clay, if I may judge from its tenacity, &c. — Yours, &c., Ernest P. Wilkins, 
F.G.S., Neiviiort." 
Films of Selenite.— (See vol. i. p. 444.)—" Thin plates of the desired thick- 
ness may be easily procured by a little tact and carcfid manipulation. Thus, 
* Our correspondent here uses the word swallow-holes with a meaning somewhat different' 
from that in which it is usually applied. Swallow-holes are the conical cavities on the surface 
of some parts of the country into which water runs either permanently or during heavy rains 
(.see Prestwich "On some Swallow holes near Canterbury," Quart. Jour.Geol. Soc, vol. x. p. 222), 
and though these swallow-holes are regarded as actual represei'.tatives of the original condition 
of many of the conical liollows, the sections of wliich are oiteii seen in the exposed chalk-strata, 
yet the latter are called sand-pipes, sand-galls, &c. 
