210 NEOZOIC TIME. [CHAP. XI. 



Geikie, 1 and in his words, " the hollow in which the shingle 

 lies is evidently the channel of an ancient stream which had 

 eroded the older basalts. At the time when this stream 

 was flowing the island of Eigg must have been joined to 

 some higher land, probably to the west or north-west, for 

 the stream brought down with it blocks of hard Cambrian 

 sandstone a rock not found in Eigg, but abundant on the 

 opposite island of Rum." Dr. G-eikie has also shown that 

 the thickness of rock removed from some of the Highland 

 valleys since the volcanic eruptions has been more than 

 3,000 feet, and that the valley in which Loch Lomond is 

 situated could not then have existed. From this we may 

 judge how much the general surface of the country has 

 been lowered since Eocene times ; and the large quantity 

 of flints in the gravels proves that large tracts of the sur- 

 rounding country consisted of chalk. It is, in fact, probable 

 (as stated in the last chapter, p. 194) that at this time 

 the whole of the central Highland region was fringed with 

 a broad, undulating mantle of chalk, and that the valleys 

 which now trench the gneissic rocks of this region were first 

 marked out by the Eocene streams which coursed over the 

 surface of these chalk plains. 



Oligocene. Deposits of this age occur only in one part 

 of England, namely, Hampshire and the Isle of Wight, and 

 only the central portion of this series now remains, so that 

 no discussion of comparative stratigraphy is required except 

 in connection with the French and Belgian series. The 

 English deposits were evidently formed in the delta of a 

 large river which drained a western continent, and they are 

 often called the Fluvio-marine series. 



The Headon group is truly fluvio-marine, having at the 

 base freshwater clays and limestones, in the middle estua- 

 rine and marine beds, and at the top estuarine and fresh- 



1 " Quart. Journ. Geol. Soc.," vol. xxvii. p. 309, and " Scenery of 

 Scotland," second edition, p. 152. 



