340 ON THE ANCIENT GRAUWACKE 
Highlands arises from the inability geologists have hitherto 
experienced of drawing, amid the perplexities of these convo- 
lutions, a base line for the whole; and from the further circum- 
stance that, for. great distances together, so completely vertical 
are the strata, that the ascending cannot be dstinguished from 
the descending direction. On visiting the Pentland range of 
hills for the first time, many years ago, there was nothing which 
so impressed me as that vertical position in which I invariably 
found the Grauwacke slates of the district. Forming the fun- 
damental rock on which all the other rocks, sedimentary or 
igneous, had been in succession cast down or erupted, I saw it 
assuming the appearance of a foundation of piles, and present- 
ing to even the very oldest of them,—the Old Red conglom- 
erate, —its upturned edges. This vertical Grauwacke, I said, 
must have assumed its present character and position, — nay, 
must have presented all its present-marks of great antiquity, — 
at a time when the materials of the conglomerate existed at 
the bottom of an Old Red Sandstone ocean, as beds of un- 
fixed water-rolled pebbles, mixed with loose sand. Nor is it 
easy, surely, to affix limits to the tremendous potency of the 
earth-tempest that must have originally raised it, over so ex- 
tensive an area, from the horizontal to the vertical position. 
Unacquainted at the time with the experiments of Sir James 
Hall, I was reminded, during my visit, of a phenomenon which 
I had witnessed when a boy, many years before, but which now 
came to assume in my memory a new character as an illustra- 
tion. A severe long protracted frost had just broken up, and 
the lower reaches of the Cromarty Frith were covered by im- 
mense floats of ice, which had formed in its upper flats and 
shallows; when one of those dead ealms which in our climate 
in the winter season so frequently herald a storm was disturbed 
by asmart breeze from the south-east, and the loose floats borne 
oceanwards by the tide were drifted back, from between the 
