TEANSACTIONS OF SECTION C. 619' 



were perfectly distinct formations, but it "would by no means follow that some 

 part of the finer beds to the south-east might not be contemporaneous with a 

 portion of the coarser beds to the north-west, as the velocity first increased, and then 

 diminished. As 1 have already said, the materials of the pebbles and of the sand 

 make it impossible to refer the main constituents to local ■ sources. Many of the 

 rocks do not exist in the Midland ; there is no reason to suppose that at that time 

 there were in this region masses of laud of sufficient area and height to feed 

 important rivers.^ From currents of any other kind we are precluded, so that I 

 believe we may safely turn our eyes northward and look for the ultimate source 

 of the Triassic sandstones and conglomerates among the older rocks of the Scotch 

 Highlands, and their extension to east and to west, though very probably the 

 materials may have been more directly supplied from Old Red Sandstone and 

 early Carboniferous strata, in remnants of which identical fragments may still 

 be seen. In like way we may regard the Trias of the south of England as the 

 detritus of at least one great river, which flowed from the west or south-west. The 

 materials of the Keuper came from the same directions in each case, but here, I 

 think, we have indications of deposition in an inland sea.. Breccias formed on 

 its coasts, and sands were at first deposited in it ; but presently the area of water 

 iocreased, and the coarser materials must have been arrested in the uplands, 

 while the fine sediment which forms the marls may have been carried out into the 

 salt lake and slowly settled down in its calm waters.- Its shoies may have been 

 hardly more favourable to a vigorous development of life than were its salt- 

 saturated waters ; during this period and the preceding Bunter the lowland border 

 of the mountains, like some of the northern districts of India, may have been arid 

 and barren regions of shifting sands. 



The Trias of Northern Scotland very probably indicates a repetition on a more 

 restricted scale of the physical conditions of the Old Red Sandstone, but after this 

 we observe signs of an encroachment of the Atlantic on the above-named old area 

 of continental land. 



The Jurassic series is represented in Northern Scotland on both the western and 

 the eastern coasts by marine or estuarine beds. This probably indicates important 

 modifications in the river channels, subsidence on the west altering the slopes, 

 reducing the length, and cutting away some of the feeding ground. Traces may 

 still be discerned in England of the two northern rivers, but that which in Triassic 

 times was the larger contributor, appears in Jurassic to have been gradually enfeebled ; 

 the other one and the south-western stream seem to have still flowed with some 

 strength. Sands, however, now become comparatively local. Probably the coarser 

 materials, as a rule, did not reach the sea. This appears at all times to have 

 been comparatively shallow and enclosed by land on every side but the south-east. 

 The recent discovery of Oxford Clay beneath the Cretaceous beds at Chatham 



' It may be iiseful to give a rough idea of the quantity of rock which must have 

 been denuded in order to obtain materials for the Bunter beds. Suppose, for 

 purposes of calculation, we consider the Bunter beds, which cover the district from 

 the Chesliire coast to the Midland counties, as forming the section of a cone con- 

 tained by two planes drawn througli the axis so as to include an angle of 30 

 degrees. If h be the height of this axis, and r the radius of the base, the volume of 



this figure is Take, for purposes of rough calculation, /t = J mile, r = 80 miles, 



■7r = 3 ; tlie volume is about 133 cubic miles. Conceive this piled up to form along 

 mound, in section an isosceles triangle 1 mile high, with a base of 4 miles. The 

 length would be over 65 miles. Thus the materials buried in the Bunter beds of the 

 above-named district represent a chain of hills unfurrowed by valleys 5,000 feet 

 high, 4 miles wide, and 65 miles long. Suppose the Pebble Bed, a like slice of a 

 cone, axis one-tenth of a mile, base 70 miles ; the volume is more than 40 cubic miles. 

 Suppose the quartz and quartzite pebbles one-tenth of its volume ; tliese represent 

 a mass of four cubic miles, or a line of hills like the above 1,000 feet high, 2 miles 

 wide, and 20 long. 



- The lake may have gradually become salt, or possibly the Muschelkalk sea may 

 have for a short space invaded Britain, and then have been insulated like the Caspian. 



