SILUEIAX KOCKS IN HERTFOEDSHIEE. 243 



its impervious nature forms as it were the bottom and embank- 

 ments of the reservoir from which our principal underground 

 supply of water is now obtained. We now come to the fourth 

 underground reservoir in descending order, the Lower Greensand 

 or Neocomian, which consists of a series of strata mostly siliceous 

 but varying much in their nature, some existing as a hard rock, as 

 the Kentish Rag, and others being loose, light- coloured porous 

 sands, forming a valuable water-bearing stratum. On one side of 

 our basin, the south, these beds are upheld by the impermeable 

 clays of the Wealden series, and on another, the north, by the 

 almost equally impei'meable clays of the Jurassic series. 



The earliest attempt to obtain a supply of water from this Lower 

 Greensand formation, in the immediate neighbourhood of London, 

 was made more than 25 years ago at a point just below the lirst 

 rise of Highgate Hill, by the side of the road from Kentish Town, 

 and the boring is known as the Kentish Town well. A well had 

 been sunk by the Hampstead Waterworks Company to a depth of 

 539 feet, passing through 324^ feet of Tertiary strata and 214^ 

 feet of the Chalk. The supply of water at that depth being in- 

 sufficient, in June, 1853, a boring was commenced in the chalk at 

 the bottom of the well, and when 430J feet of chalk had been 

 passed through, giving to the Chalk a total thickness of 645 feet, 

 the Upper Greensand was entered. This was found to be 1 3^^ feet 

 thick, and to be underlaid by the Gault with a thickness of 130i 

 feet, and with the usual layer of phosphatic nodules at its base. 

 So far, to the total depth of 1 11 3A- feet, the strata were found to 

 be in regular succession, and it had been anticipated that at this 

 point the next bed in regular descending order, the Lower Green- 

 sand, would be entered. Such, however, was not the case. A 

 series of beds of sandstones and clays, and a peculiar con- 

 glomerate, were passed through for 188^ feet, making a total depth 

 of 1302 feet. When this point was reached, the property came 

 into the possession of the jS^ew River Company, and the boring 

 was not carried to any greater depth. The geological age of the 

 strata below the Gault could not be satisfactorily determined, 

 owing to the method of boring rendering it uncertain whether the 

 few fossils obtained from these beds were really derived from them 

 or had fallen down the bore-hole from higher beds, a supposition 

 which the fossils themselves favoured, being species which were 

 only known to occur elsewhere in beds of Gault or Upper Green- 

 sand age. Mr. (now Professor) Prestwich inclined at the time to 

 the opinion that in their mineral character the Kentish Town beds 

 closely resembled the red marls of the Xew Red Sandstone group,* 

 and subsequently came to the conclusion that they " should 

 probably be referred to the Old Red Sandstone, "f 



* "On the Boring: through the Chalk at Kentish Town, London." 'Quart. 

 Journ. Geol. Soc' vol. xii, p. 6. — 1856. See also the paper by Mr. Prestwich 

 '' On the Boring through the Chalk at Harwich." lb. vol. xiv, p. 251. — 1858. 



t ' Report of the Commissioners appointed to enquire into the several matters 

 relating to Coal in the United Kingdom,' vol. i, p. 156. — 1871. 



