( 913 ) 



XXV. — 071 the Diatomaceee in the Llyn Arenig Bach Deposit. 

 By Henry Stolterfoth, M.D. 



iBead 13th October, 1880.) 



Having been for some time past engaged in examining diatoma- 

 ceous deposits, most of them from foreign countries, many from 

 places one has small chance of visiting, it was with no little interest 

 I heard of the discovery made by Mr. W. F. Lowe, of the Arenig 

 Bach deposit. I only know of one other diatomaceous deposit in 

 North Wales, mentioned by Smith in his ' British Diatomaceae ' as 

 Dolgelly earth, but which I have obtained, though only in small 

 quantities, and from some feet below the surface of the water, at a 

 lake called Cwm Bychan, fifteen miles from Dolgelly. 



In October 1879,1 accompanied Mr. W. F. Lowe and a few friends 

 to the Arenigs. We had a wet day, and lost some time in finding 

 the lake, which lies nine miles from Bala, amidst the mountains. 

 On reaching it, we found that it had been drained to the extent of 

 about twelve feet below its normal level, so that about one-third of 

 its surface was dry. The edge of the lake for about ten yards 

 consisted of stones from the surrounding mountains, which are of 

 igneous origin. The remainder of the uncovered surface that we 

 were able to examine, was covered with about one foot of peat, and 

 under this was the diatomaceous material, also about one foot thick. 

 A small stream at the head of the lake had cut a section through 

 the peat and diatoms, and the latter rested directly on the rocky 

 bottom. This partial inspection of the bed of a mountain lake, 

 more than repaid the trouble of getting to it, for not only is it 

 something to have seen a deposit in situ, and to have handled and 

 examined it in more than microscopic quantities, but it enabled me 

 to get my specimens from different positions as naturally deposited, 

 an important point if we are ever to determine the time during 

 which a deposit has been forming, and the changes that may have 

 taken place with regard to species. 



This much I think we may say with regard to the Arenig 

 deposit, that the diatoms have been collecting at the bottom of the 

 lake ever since the last glacial period, and although the deposit was 

 only a foot thick where we examined it, it is probable that in the 

 centre of the lake it is much thicker. 



It now remains for mo to speak more pai-ticularly of the 

 diatoms. 



For the purpose of a systematic examination, I made, 



1st. (liatliorings of as many growing forms as I was able, in 

 and about the lake. 



2nd. I took some of the peat immediately above the deposit. 



3rd. A portion of the de}X)sit Ix^Iow tlie poaf. 



