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XVIII. — A Diatomaceous Deposit from North Tolsta, Lewis. By John 

 Rattray, M.A., B.Sc, of H.M. "Challenger" Commission, Edinburgh. 

 (Plate XXIX.) 



(Read May 2, 1887.) 



The sample of the remarkable deposit of Diatomite, upon which the sub- 

 joined observations have been made, was forwarded to me some time ago by 

 William Morrison, Esq., of the Academy, Dingwall, through my friend Mr 

 John Gunn. With the exception of a brief notice in the newspapers soon after 

 the discovery of the deposit by Mr Morrison a few months ago, and a paper 

 entitled " On some New Localities for the Mineral Diatomite, with Notes on 

 the Chemical Composition of the Specimens exhibited," in the Mineralogical 

 Magazine and Journal of the Mineralogical Society, vol. vii. No. 32, pp. 30-34, 

 July 1886, by W. Ivison Macadam, F.C.S., F.I.C., Professor of Chemistry, New 

 Veterinary College, Edinburgh, no statistics have been published in connection 

 with the deposit, nor has any account hitherto been given of the Diatomacecs 

 which play so prominent a part in its composition. 



Mr Morrison""" has made the following communication to me with respect 

 to the deposit itself, and to the general features of the district in which it is 

 found : — " The sample of Diatomite sent is from the bottom of a drainedt fresh- 

 water loch (Loch Osabhat), North Tolsta, within the civil parish of Stornoway. 

 The area of deposit is about 1^ acres, and is covered with a skin of peaty soil 

 of about 3 feet thick. I found an average depth of 7\ feet for the pure deposit 

 over this area. The Diatomite is found pure to the rocky bottom of the loch. 

 I examined sections of it in the trenches for draining cut through it, and saw 

 no appearance of underlying deposit. The trenches, of course, did not go to 

 the bottom of the deposit in the middle or deepest part, but, so far as I can 

 judge, the Diatomite was pure and unmixed to the very bed-rock of the loch. 

 In mass and in situ the deposit, when cut into, is bluish and foetid, but when 

 dry both smell and colour disappear. The rocks of the district are of the 

 coarsely granular gneiss of the island, alternating with argillaceous schists. 

 The rock around the loch is gneiss, much kaolinised. North of the conglome- 

 rate junction at Gress and on to North Tolsta the rocks are argillaceous schists 

 of a yellowish-green colour. I was struck by the large number of jaspery pieces 

 of gneiss I met on the road to the loch, and quite as much struck by the 

 extreme kaolinised appearance of the rocks in the immediate neighbourhood 



* Letter, dated April 20, 1887. 



t From Professor Macadam's paper it appears that this loch was drained in 1874. 



VOL. XXXIII. PART II. 3 Q 



