226 Froccedings of the Royal Pliysiccd Society. 



at this point runs over gravel. There is no exposure of the 

 conglomerate of the Old Eed Sandstone (the " Great Con- 

 glomerate " of my father's writings), which probably forms 

 the underlying rock. The place is apparently one of these 

 opener basins common on our older valleys where the hollow 

 is thickly lined with boulder-clay, and is easily cut into 

 river terraces, each covered as usual by a sheet of river- 

 gravel. One raw scar of grey boulder-clay showed, like the 

 mark of a fresh landslip, on the opposite bank ; traces of 

 the terrace gravel of the mountain stream could be seen near 

 the water's edge. The remnants of the bog surround the 

 walk which encircles the pond. 



The peat is " young," or raw and imperfectly formed : the 

 vegetation forming it appeared to be still in an actively 

 decomposing condition: the pools w^ere clouded with hydrous 

 oxide of iron, like a suspended sediment of burnt sienna. A 

 string of bubbles rose now and then from the bottom of the 

 pond, which, when I saw it, had become green with grassy 

 weeds. It was plain that when prevented from breaking at 

 the surface by a film of ice the bubbles had been frozen-in — 

 imprisoned, and perhaps compressed, by the formation of 

 new ice underneath them. 



It would have been of interest to obtain samples of this gas 

 for analysis, and my friend. Professor Ivison Macadam, has 

 furnished me with an apparatus for the purpose, at present 

 placed in the hands of Mr Morrison, the well-known rector of 

 the Dingwall Academy, who has kindly undertaken, subject 

 to the consent of the proprietor of the castle, to visit the spot 

 for the purpose. But since my visit in 1888 there has been 

 no skating ; and it is possible that, as supposed by observers 

 on the spot, the exhalations may be exhausting themselves. 



The explosive character of the gas when issuing into the 

 air identifies it as largely consisting of marsh gas, — the 

 explosive fire-damp of the coal pits, — the cause of what is 

 known as singing coal, and of blowers in petroleum springs. 

 Mingled with it are doubtless other gases — nitrogen, carbonic 

 acid (unless absorbed by the water), perhaps some olefiant gas, 

 and probably a little sulphuretted hydrogen (which, however, 

 would be absorbed at once), which I have observed to be 



