516 Geological Society. 



consequence of the influx of distinct materials, but in the author's 

 opinion of the grinding down by the ice of the mud and other 

 detritus. 



Mr. Lyell then describes the phsenomena 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 in 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, 

 which would become lakes or peat-mosses if the till were extracted. 

 This distribution of the detritus, Mr. Lyell observ^es, may be ex- 

 plained on the supposition that, if the cold period came on slowly, 

 the advance of the glaciers would push forward the detritus accumu- 

 lated at their termination, and fill up, wholly or in part, the lakes or 

 other cavities which they would encounter in their progress. Along 

 most of the river- courses, and in the lowest depressions of Strath- 

 more, the till is covered by stratified sand and gravel. 



One of the most remarkable peculiarities of the transported ma- 

 terials of Forfarshire and Perthshire is a continuous stream, from 

 three to three and a half miles wide, of boulders and pebbles, trace- 

 able from near Dunkeld, by Coupar, to the south of Blairgowrie, 

 then through the lowest part of Strathmore, and afterwards in a 

 straight line through the lowest depression of the Sidlaw Hills from 

 Forfar to Lunan Bay, a distance of thirty-four miles. No great river 

 follows this course, but it is marked everywhere by lakes or ponds, 

 which aiFord shell-marl, swamps, and peat- mosses, commonly sur- 

 rounded by ridges of detritus from fifty to seventy feet high, con- 

 sisting in the lower part of till and boulders, and in the upper of 

 stratified gravel, sand, loam and clay, in some instances curved or 

 contorted. The form of the included spaces is sometimes oval, 

 sometimes quadrangular. The finest examples are in the lower 

 tract, which has the Dean for its southern boundary, and the road 

 from the bridge of Ruthven to the south of the grounds of Lindertis 

 for its northern. The Grampian boulders are throughout the same ; 

 but there are associated with them masses of actinolite schist, which 

 Mr. Blackadder has ascertained could be derived only from the val- 

 ley of the Tay. The fragments of secondary rocks belong to the 

 formations of the districts in which they occur. Though the country 

 occupied by these marl-loch lakes is not traversed longitudinally by 

 any river, yet it is so low, that if the transported matter were re- 

 moved, a very slight depression would cause the sea to flow from 



