CHAP. IV.] SILURIAN PERIOD. 43 



rate about seventy-five feet thick, consisting of rounded 

 pebbles and boulders in a matrix of purple sand, and it 

 passes up into soft sandstones 250 feet thick. In another 

 locality these beds are absent, and rocks belonging to a 

 higher horizon rest unconfonnably on the Ordovician ; 

 again there is a basal conglomerate about fifty feet thick 

 and greenish in tint, " the main mass of this conglomerate 

 is made up of well-rounded boulders varying from one inch 

 to a foot and a half in diameter ; they consist of pieces 

 of granite, porphyry, felstone, greywacke, shale, Lydian- 

 stone, quartz, and jasper, embedded in a coarse sandy 

 matrix of a dark-green colour, and excessively.indurated." ] 

 Higher still is a band of quartz conglomerate succeeded by 

 a series of shales, grits, and flagstones, and the total thick- 

 ness of the Valentian here is estimated at about 1,800 feet 

 (exclusive of the Cyrtograpsus shale). 



It is clear, therefore, that land existed in this G-irvan 

 district at the commencement of Silurian time, and that in 

 passing southward and eastward we recede from this land ; 

 the mass of the land must consequently have lain to the 

 north and north-west. 



The higher Silurian groups are represented in the south 

 of Scotland by the Hawick and Kiccarton Beds, with their 

 equivalents, the Bargany and Stratton groups, in Ayrshire, 

 and finally, in Lanark and Edinburgh there is a series of 

 brown and grey beds with Wenlock and Ludlow fossils, 

 which are from 3,000 to 4,000 feet thick, and pass up into 

 the Lower Old Red Sandstone. We may conclude, therefore, 

 that a thick mass of Silurian strata once extended over the 

 whole area of the Southern Uplands, and passed beneath 

 the Lowlands of central Scotland. No Silurian rocks, 

 however, emerge from beneath the Old Eed Sandstone on 

 the northern border of the Lowlands, where this sandstone 

 rests unconformably on the Highland gneissic series; neither 

 1 Lapworth, " Quart. Journ. Geol. Soc.," vol. xxxviii. p. 638. 



