NOTES AND QUERIES. 



175 



tooth is smaller ; its leng-th on the grinding-surface eight and a half inches, by 

 three inches wide in the centre, tapering ofl" to one inch at each end ; it has 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 

 Marl — a mixture of clay and sand, — whether above the Kenper Sandstone or 

 below it, camiot be made into bricks or tiles, and the fine 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 down in another. They are thus found in most Triassic districts as basins of 

 clay of unecpial depth and extent, and as beds of sand ; it is in these beds and 

 basins, and in the gTavel-pits, that the young geologist may expect to find 

 mammahan remains. The geological maps of the Ordnance Survey, although 

 generally very acciurate in the boundaries of formations, at least wherever I have 

 examined them, are still defective in one particular and calculated to mislead. 

 Miles of smface are laid down as Red Marl, 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 grouped together ; but such a map would at least greatly 

 jfacilitate the search for Mammalian remains. — J. Plant, Leicester." — On the 

 large diagram-map just executed under my direction, the com-se and extent of the 

 Great-Northern JDrift is laid down as far as our present knowledge extends. 

 Additional information is still, however, very desirable. — Ed. Geologist. 



Sand-pipes near Swainstone, Isle of AVight. — " Dear Sir, — In accord- 

 ance with the wish of jVIr. Prestmch, 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 Sir John Simson, Bart. Its 

 interest chiefly arises from its bearing on the date of the formation of swallow- 

 holes,* in conjunction with the period of the uplieaval of the vertical chalk-strata 

 which are well known to form a belt through the Isle of Wight. At the spot in 

 f[uestion 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 escarpment of tlie central 

 belt of chalk-liills. Tiie uppermost beds of the chalk are divided at first 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 cutting 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 diflering from those ordinarily 

 . met with. In no instance can I find flints which can be considered as water- 

 worn ; for the most part they are fractured, and some are of very large size. In 

 most of the swallow-holes flints may be seen fining, as it were, the circumference ; 

 while the centre is composed of clayey gravel. In those nearest the eastern extre- 

 mity of the section clay predominates, having a great similarity to the Plastic 

 Clay, if I may judge from its tenacity, &c. — Yours, &c., Ernest P. Wilkins, 

 F.G.S., Newport." 



Films of Selenite.— (See vol. i. p. 444.)—" Thin plates of the desired thick- 

 ness may be easily procured by a little tact and careful 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.GeoI. Soc,,vol. x. p. 222), 

 and though these swallow-holes are regarded as actual representatives of the original condition 

 of many of the conical hollows, the sections of which are often seen in the exposed chalk-strata, 

 yet the latter are called sand-pipes, sand-galls, &c. 



