341 
siderable thickness of unstratified matter full of Grampian boulders, 
and covered for the greater part with stratified gravel and sand. In 
some cases the latter exhibit the diagonal laminze common in sub- 
aqueous formations; and in others the strata are so contorted, that 
a perpendicular shaft might intersect the same beds three times. In 
the latter instances the ‘surface of the subjacent red boulder clay 
has not partaken of the movement by which the stratified deposit 
was contorted; and in consequence Mr. Lyell ascribed the effect, 
when he first beheld it in 1889, to the lateral pressure of large 
masses of drifted ice repeatedly stranding upon a shoal of soft ma- 
terials*. In the middle of the tract between the South Esk and the 
Proson is a dry valley, and to the south of this valley, near the Pro- 
son, an excavation was made ten years ago, which exposed extremely 
contorted beds overtopped by others perfectly horizontal, having 
been formed by tranquil deposition after the disturbance of strata 
previously deposited. The phenomena exhibited by the till in this 
district, Mr. Lyell conceives, might bewell accounted for by supposing 
the union of three or four large glaciers; but he considers it difficult 
to explain the accumulation of the overlying stratified materials, the 
top of which must be 600 feet above the level of the sea, and facing 
the Strath. In following out the narrow ridge which intervenes 
between the Proson and the Carity, during last October, in company 
with Dr. Buckland, the latter drew the author’s attention to a spot 
half a mile south-west of the House of Pearsie, where the surface of 
a porphyry rock was polished, furrowed, and scratched. The quar- 
rymen of Forfarshire also state as a general fact, that rocks of suffi- 
cient hardness, when first laid bare, are smooth, polished and scored; 
and Mr. Blackadder has found on the Sidlaw Hills large boulders of 
sandstone grooved and polished. Another general fact mentioned 
by Mr. Lyell is, that the unstratified boulder-clay becomes more and 
more impervious in the lower part of the Grampian glens, not in 
consequence of the influx of distinct materials, but in the author’s 
opinion of the grinding down nny the ice of the mud and other 
detritus. 
Mr. Lyell then describes the phenomena of the second district, 
or Strathmore. Though this district may be considered as one 
great strath, yet it is divided into many longitudinal ridges and. 
valleys. The former, sometimes 300 feet in height, are for the 
greater part parallel to the strike of the old red sandstone, and are 
generally covered to the depth of sixty or more feet with till and 
erratics, derived from the Grampians and the subjacent strata. This 
covering is so general, that the structure of the district can be de- 
tected only im the ravines through which the principal rivers pass. 
The till constitutes invariably the oldest part of the detritus. The 
boulders which it contains sometimes exceed three feet in diameter: 
on the north muir of Kerriemuir is a block of trap-rock, six feet by 
five feet, and near it is a mass of mica-schist, nine feet long by four 
feet wide and three high. The till has been ascertained by Mr. 
Blackadder to fill, in many places, deep hollows in the sandstone, 
* See ante, p. 178. 
