382 Transactions of tlie Society. 



looking for them as the lightest particles, and it need scarcely be 

 said, always in vaiu. 



In 1879, Dr. Bossey, F.E.M.S., of Eedhill, to whom I am 

 greatly indebted for assistance in the study of diatoms, encouraged 

 me to renew the search : and I again examined samples of London 

 clay from various localities, and from the whole length of the 

 Sheppey cliffs. This I did by drying the clay, mixing it with water, 

 and then looking for the diatoms on the surface, but none were seen. 



I was one evening looking for the first time at a slide of Aula- 

 codiscus Oreganus, when the thought struck me that the metallic 

 specks noticed in 1877, and almost forgotten, were the very things 

 I was in search of. I quickly turned out all the samples of clay 

 from the Sheerness well that I had preserved ; but no diatoms were 

 visible. I then disintegrated and washed some from about the 

 same depth as that at which the metallic specks had been seen, and 

 after several failures I ultimately, on December 1st, 1879, found 

 the shining disks that had previously attracted my attention. 



Bearing in mind that the objects were evidently mineralized, 

 and therefore heavier than the fine particles of clay, I got rid of 

 as much of the clay as possible, and found that the residuum con- 

 sisted almost entirely of the mineralized forms more or less entire, 

 and in some cases aggregated in small masses. 



I had in my possession a complete set of specimens of clay 

 taken from a well in Sheerness dockyard in 1812, and these I 

 broke up that I might ascertain if the diatoms occurred in that well 

 at the same or at any other depth. The result was that I found 

 them, but only at about the same horizon as those in the Town 

 weU. I then subjected to close examination clay from a boring 

 close to the old " King's Well" (which was commenced in 1781, 

 and was the first well that pierced the London clay in the Isle of 

 Sheppey), and found the diatoms still at the same level. 



Finding them thus in three wells some distance apart, I sus- 

 pected that, although limited to one zone, there was probably a 

 considerable lateral extension of the diatomaceous band. To test 

 the correctness of this conclusion, I went to Heme Bay, and 

 going eastward from the town along the beach, examined the 

 clay at intervals, which, owing to the inclination of the beds, 

 represented a considerable vertical thickness. Near Oldhaven Grap, 

 the basement bed of the London clay crops out. Just before getting 

 to this spot, on splitting open a clod to get an unweathered surface, 

 and using a lens, I was much gratified by finding the diatoms. 

 Further examination showed that they occur in irregular patches, 

 which had helped to produce the slight lamination observable in 

 the clay there. 



At Upnor, as is well known, the junction of the London clay 

 with the beds below is clearly shown in the most easterly pit. Here 



