420 MR JOHN RATTRAY ON A 



of the loch itself. In the middle of the drained area is a ' crannoge,' the only 

 fertile spot within the area, built on piles driven into the deposit of Diatomite, 

 and overlaid by flat stones, over which lies the soil of the 'crannoge.' A 

 causeway on piles connected this ancient lake-dwelling with the shore. Frag- 

 ments of unglazed pottery and deer's horns were found when trenching around 

 this little island. It may be of collateral interest to mention that directly south 

 from Tolsta, across the Broad Bay, at Garrabost, is a deposit of black boulder 

 plastic clay, now and for years back worked into bricks, at the 100-feet level. 

 The deposit of boulder clay has in it fragments of recent and boreal shells. 

 The average height of Loch Osabhat above the sea is 300 feet, the shore is 200 

 yards distant from the loch, and the coast is precipitous." 



The colour of the dried material is light drab, and its texture is open and 

 porous. Save at the margins, where mineral particles of considerable size— 

 from 3 to 5 mm. in diameter— are to be found, but few minerals occur through- 

 out its mass, the average size of such as do occur being in the specimen 

 examined from 01 to 015 mm., or even somewhat greater, in diameter. 



The results of an analysis* of the deposit have shown it to contain 13*875 

 per cent, of organic matter, consisting of chlorophyll, cellulose, &c, and 86125 

 per cent, of inorganic constituents, the analysis calculated free from moisture, 

 giving 13*874 per cent, of organic and 86125 of inorganic components. The 

 inorganic portion has yielded 1*326 parts of ferric oxide, 0*753 parts of aluminic 

 oxide, and 94*495 parts of silica, mostly in the form of Diatoms. 



The weight of a cubic foot of the Diatomite is said to be 54 lbs. 



The following general features may be pointed out in connection with this 

 deposit : — 



(1) As indicated above, its frustules are entirely of fresh-water origin. 



(2) As in external appearance, so in microscopic character, it exhibits a 

 great degree of uniformity throughout (a) with respect to number of genera 

 represented, which are few, and (/3) with respect to their ratio to one another. 

 Thus the preponderance of Navicular, Epithemiw, Eunotiw, and Surirellce is to 

 be contrasted with the much less frequent presence of Cymbellce, Encyonemw, 

 Synedrce, Fragilarice, Tabellariw, Cocxoneidce, and Cydotella. 



(3) Among the heavier material Navicular and Surirellce predominate, 

 whilst among the lightest minute forms of Navicula, Cymbellce, Fragilarice, 

 and Cydotella are most common. 



(4) The rare occurrence of some species, even of large size, is remarkable, 

 e.g., Gompkonema geminatum, Ag., var. bipunctata, nov., and contrasts curiously 

 with the great abundance of other species of the same genus, e.g., Gompkonema 

 araminatum, Ehrenb. Among the rarer species the following may also be 

 noted: — Encyonema ren/ricosum (Kiitz.), Grim. ; Navicula mesolepta, Kutz. ; 



* Macadam, op. cit., pp. 33, 34. 



