A SUMMER RAMBLE AMONG THE HEBRIDES. 201 



blue clays alternate with bands of cream-coloured, light-green, 

 and dark-blue limestones; and over all there rests a band 

 of the red boulder-clay, capped by a thin layer of vegetable 

 mould. It is a curious circumstance, well fitted to impress 

 on the geologist the necessity of cautious induction, that the 

 boulder-clay not only overlies, but also underlies, this fresh- 

 water deposit ; a bed of unequivocally the same origin and 

 character with that at the top lying intercalated, as if filling 

 up two low flat vaults, between the upper surface of the Corn- 

 stone and the lower band of the Weald. It would, however, 

 be as unsafe to infer that this intervening bed is older than 

 the overlying ones, as to infer that the rubbish which choaks 

 up the vaulted dungeon of an old castle is more ancient than 

 the arch that stretches over it. However introduced into 

 the cavity which it occupies, whether by land-springs or 

 otherwise, we find it containing fragments of the green and 

 pale limestones that lie above, just as the rubbish of the 

 castle dungeon might be found to contain fragments of the 

 castle itself. When the bed of red boulder-clay was inter- 

 calated, the rocks of the overlying Wealden were exactly the 

 same sort of indurated substances that they are now, and 

 were yielding to the operations of some denuding agent. 

 The alternating clays and limestones of this outlier, each of 

 which must have been in turn an upper layer at the bottom 

 of some lake or estuary, are abundantly fossiliferous. In 

 some the fresh-water character of the deposit is well marked : 

 Cyprides are so exceedingly numerous in some of the bands, 

 that they impart to the stone an Oolitic appearance ; while 

 others of a dark-coloured limestone we see strewed over, like 

 the oozy bottom of a modern lake, with specimens of what 

 seem Paludina, Cyclas, and Planorbus. Some of the other 

 shells are more equivocal : a Mytilus or Modiola, which 

 abounds in some of the bands, may have been either a sea or 

 a fresh-water shell ; and a small oyster and Astarte seem de- 



