Boulder Formations and Erratic Blocks. 325 



level, particularly on the west side, where the faces of these 

 almost vertical cliffs of adamantine hardness are scored hori- 

 zontally, as potter's clay might be by the pressure of the fin- 

 gers, or like the moulding of a cornice by the plasterer's tool.*" 



The question naturally arises, at what period were these val- 

 leys in Dumbartonshire and in Skye occupied by glaciers \ That 

 they were so after the land had been formed into the present 

 mountains and valleys is obvious ; but that defines no particu- 

 lar period. We have in the Gare Loch two distinct classes of 

 phenomena, which could not have been produced either by the 

 same agents or at the same time. We have proof of the 

 action of sub-aerial glaciers ; we have also proof that there are 

 erratic blocks that could not have been brought into their pre- 

 sent position unless the ground on which they rest had been 

 submerged : they were dropped, it is most reasonable to sup- 

 pose, from icebergs floating in a sea, and arrested by elevations 

 in the sea-bottom. During such submergence there could be 

 no glaciers in the valleys of Gare Loch or Coruisk. Are we 

 to suppose that after these valleys had been occupied by a gla- 

 cier, and the erosions had been made, the land sank down, 

 continued for a long interval as a sea-bottom, during which 

 time the glaciers melted away, and that the land again emerged, 

 bearing the erratic blocks upon it \ The subject is one of vast 

 difiiculty ; but the phenomena evidently involve great changes 

 in the condition of the land, and consequently, perhaps, in the 

 climate of that region. 



It is an important feature in the history of the boulder for- 

 mation, that the mode of its accumulation, and the direction 

 of the channels, furrows and striae worn in the rocks, indicate 

 a force coming from the north, between NW. and NE. The 

 worn and polished surfaces of so many rocks facing the north, 

 while their rugged unworn surfaces point to the opposite direc- 

 tion, are farther proofs of the same movement. The travelled 

 rounded boulders and detritus from the middle of Sweden and 

 Norway southward, must therefore have been derived from 

 land existing north of that latitude. 



Submarine currents are by many geologists supposed to have 

 been the moving power ; and it is also said, that the detrital 

 matter they hurried along smoothed and polished the rocks 



VOL. XLI. NO. LXXXII. — OCTOBER 1846. Y 



