190 GEOLOGY OP ARRAN. 



conditions affecting the marine creatures prevailed over this 

 district. But the area is too limited to warrant any general 

 conclusion. The probable conditions have been already 

 noticed (Arts. 20-22). The boulder-clay may have been 

 formed by laud ice; the shell-bed over it, under and around 

 a rim of ice when the land had been depressed; and the 

 upper drift beds partly by local glaciers as the ice was 

 disappearing, and partly perhaps by rivers, then of larger 

 volume than now, as already hinted. The elevation of the 

 shell-bed, or of the upper surface of the boulder-clay (Art. 

 21, last sentence, in which this idea is obscurely expressed), 

 taken in connection with the depth due to the species, 

 gives us the measure of the depression of the land. Now, 

 the elevation of the beds in question ranges from 70 to 

 180 feet; so that the greatest depression of which we have 

 any evidence here is 180 feet below the present sea level, not 

 taking account of the depth required for certain species ; and 

 we hesitate to speak with confidence of any greater depres- 

 sion, with our present knowledge of the beds farther up the 

 glens. In Clachan Glen there are beds of at least equal 

 altitude and enormous thickness ; but their altitude has not 

 yet been determined. 



91. The bed and banks of these four streams present many 

 objects of interest besides the shell-beds. They exhibit sand- 

 stones and shales with nodular limestone, and sometimes 

 nodular ironstone, among which no fossils have yet been 

 found, and these rocks are pervaded by traps and felstones 

 of all varieties, both in dikes and overlying masses. Near the 

 head of Crook-crever burn claystone appears in both relations : 

 on the bank it overlies, in the bed of the stream it breaks 

 through the sandstone in conformable beds, and produces a 

 very decided change upon it, altering it to the condition of a 

 fine white quartzite, a very clear proof that the claystones in 

 many places are not altered sandstones, but themselves 

 erupted rocks. The bed of the Slaodridh at Glen Rie mill, 

 and the country thence to Lag, and again far up the stream 



