370 W. H. HUDLESTON ON THE CHEMICAL 



any way sufficient. The large proportion of phosphate of lime pre- 

 sent in the Bala rocks is probably due to their containing, like the 

 " Menevian" rocks, so large a number of Trilobites — these two for- 

 mations containing them in greater abundance than any others. 



Secondly, though it is possible that' some of the more recent 

 deposits may have derived a part of the phosphate of lime contained 

 in them from apatite dissolved out of eruptive rocks, as recently sug- 

 gested before this Society, yet on the other hand it is even probable 

 that eruptive rocks derived phosphate of lime from the earlier sedi- 

 mentary rocks through which they passed, and where it had previously 

 been deposited by animal and vegetable life. We know of no 

 eruptive rocks in this country of so early a date as the time when 

 these " Menevian " beds were deposited. The dykes, which in 

 some cases appear to run almost in the line of the bedding of these 

 older Cambrian rocks throughout Wales, always alter the beds on 

 both sides; and I do not think that any of them are of older 

 date than the Upper Arenig or Lower Llandeilo epoch, in which 

 we for the first time in the succession meet with truly contempora- 

 neous tuffs and ashes. In Canada, however, it is said there are erup- 

 tive rocks of Laurentian age ; but whether these contain phosphate 

 of lime I do not know. The source from which the P 2 5 was first 

 obtained must of course still remain a difficulty, and we can only 

 expect at present to unravel the mystery so far as the facts laid open 

 before us in nature enable us clearly to do so. We can hardly do 

 more than inquire into the processes by which this ingredient, so 

 indispensable now to animal and vegetable life, was first separated 

 from the sea, in which from the very earliest period it seems to have 

 existed in a state of solution, and the means by which it was after- 

 wards deposited in such large proportions in some sedimentary rocks. 

 In an agricultural point of view the presence of so much phosphate 

 of lime in some of the series of beds, as " Menevian " and " Middle 

 Arenig," must be a matter of considerable importance ; and I find, 

 on looking over the geological map of St. Davids, that where these 

 series occur they underlie the very richest land in that neighbour- 

 hood. The Middle Arenig series particularly shows this ; for it is 

 of greater thickness and extends in a more continuous line than the 

 " Menevian," and this line from time immemorial has had a fame 

 for the goodness of its produce, whether in stock or cereals. 



Appendix. On the Chemical Analyses of the Hocks. 

 By W. H. Httdleston, Esq., M.A., F.G.S., F.C.S. 



At the request of Mr. Hicks I made several analyses of Cambrian 

 rocks, principally with the view of determining the amount of phos- 

 phoric acid in each ; but incidentally, in one or two cases, an analysis, 

 more or less complete, has been made of the specimen of rock itself. 



ISFo. I. is a darkish grey flaggy rock, with an impression of Para- 

 doxides Davidis on one side. Portions of the slab immediately 



