1857-] TRIMMER — BOULDER-CLAYS. 173 



remains, we must look to groups rather than to individual species. 

 The intercalation of beds of shells in situ in the midst of the boulder- 

 clay is not a solitary fact — similar deposits have been described in 

 other localities. Neither is it at variance with the analogies of exist- 

 ing arctic seas, where, amidst a general absence of shells, there are 

 instances of their occurrence, as in Baffin's Bay, described by Dr. 

 Sutherland. As a general fact, the boulder-clay forms the lowest 

 part of the erratic tertiaries. It is, however, occasionally intercalated 

 with masses of sand and gravel, more particularly near the mouths 

 of rivers. 



I may here mention another instance of an upper and lower 

 clay separated by a mass of sand. In the valley of the Weir, near 

 Durham, the late Professor Johnston, in his ' Agricultural Che- 

 mistry and Geology/ described an upper and a lower clay separated 

 by sand. On visiting the district, in company with my lamented 

 friend, I found the river flowing through a deep mass of sand. At 

 some little distance this was covered by boulder-clay, under its usual 

 form. I was conducted to two sections where there appeared to be 

 boulder-clay below the sand. I could not, however, satisfy myself 

 that they were not landslips. The sections at the neighbouring 

 collieries, where records are kept of all the beds sunk through, 

 whether belonging to the coal-measures or to the superficial deposits, 

 would clear up this point. 



The passage of the oolitic erratics across the chalk-ridge near 

 Swaffham is analogous to the passage effected by the Cambrian 

 erratics across the Pennine chain at the Pass of Stainmoor, whence 

 they have been distributed down the valleys of the Tees and Humber, 

 and lodged on the summits of the oolite-cliffs at Scarborough and 

 the chalk- cliffs of Flamborough Head. It is also analogous to the 

 passage of the Lickey pebbles across the Cotswolds. In each case 

 the different lines of erratic materials held their southern course 

 separately so long as the dividing ridges were above the glacial sea ; 

 but when their cols or water-sheds became sufficiently submerged, 

 the erratic blocks crossed them, and the blending lines continued 

 their southern course so long as the more distant rocks from which 

 they were derived were not too much submerged to continue to 

 furnish materials. 



One fact I would urge on the attention of those who see the 

 effects of a glacier wherever they meet with scratched rocks and 

 scratched detritus ; it is, that in the erratic deposits of Norfolk, 

 where we cannot invoke the presence of glaciers in the ordinary 

 sense of the term, the local fragments derived from the shales of 

 the Kimmeridge Clay and from the hard Chalk have the peculiar 

 form and scratched surface which pervade the local detritus of our 

 Alpine districts. 



That we have evidence of glaciers in the valleys of Wales and 

 Cumberland, as pointed out long ago by Buckland, and confirmed 

 by Davison, is undeniable. It is undeniable also, as suggested by 

 the latter, that they have had a large share in clearing the boulder- 

 clay out of the principal valleys. We have evidence, too, of the 



