384 Transactions of the Society. 



in the centre of the glass, and draw up some of the water with such 

 particles as will rise with it. Blow this out of the pipette on to a 

 slide or into another watch-glass, and dry off the water. The 

 partially transparent diatoms will be found chiefly on the surface." 

 These light forms are not nearly so abundant as the more 

 thoroughly mineralized ones, and are not always to be found in 

 clay containing the others. 



With the diatoms were found Sponge spicules, Xanthidia, Radi- 

 olaria, internal casts of Foraminifera,* and some interesting forms 

 whose affinity has not yet been determined. 



From various considerations that need not be stated, there is 

 little doubt that these diatoms existed continuously througbout the 

 London clay period. Yet, as far as my own observations have gone, 

 and from what I have learned from others, no trace of them is 

 found except as having existed in the early part of that period. 



I trust that the absence of diatoms, either mineralized or in 

 the ordinary siliceous condition, from the whole of the London clay 

 above the zone referred to, will be satisfactorily accounted for. I 

 can only indicate the problem that awaits solution. 



Among the large number of letters respecting these diatoms 

 that have reached me from nearly all parts of the world, perhaps 

 one from Count Castracane is the most interesting, and I therefore 

 append an extract from it : — 



" You could not have given me a greater pleasure than sending 

 a sample, and giving me notice of your most interesting discovery 

 of a deposit of diatoms belonging to the Lower Eocene. But the 

 appearance of these diatoms is so novel, that I preferred first to 

 make a careful study of them before giving any description. I must 

 confess that at the first examination I could not persuade myself 

 that a powder like this, dark and heavy, and with parts of metallic 

 lustre, could really contain diatoms. The great number of diatom- 

 aceous deposits known to us, always appear as a white, light 

 material, and the specimens are transparent. I dare say that I am 

 not mistaken in saying that you have been the first to discover dia- 

 toms, not only fossil, but mineralized, in which the organic parts are 

 found replaced completely (atom by atom) by pyrites. This latter 

 substance has been deposited by an electro-chemical process so as 

 to form the impression. To be able to prove this point, it might 

 be of some advantage to examine the matrix with the diatoms in 

 situ, and therefore you will much oblige me by sending some of the 

 clay containing them. It will also be interesting to ascertain if 

 the same types of diatoms in the condition of silicon are to be met 

 with in the same stratum, or in the next formation to it. Also, if 

 they are to be found living in the water of the neighbourhood. I 



* At higher levels in the clay where the diatoms are not fonnd, entire .-hells 

 of Foraminifera, partially or quite filled with pyrites, are plentiful. 



