82 GEOLOGY OF ARRAX. 



on the shell-beds at once. They rise steeply to a great 

 height on the east bank of the stream, a very little way to 

 the north of the upper end of the gorge. Fragments of 

 shells on the surface soon shew that we are in the right 

 place. In an excursion to these beds, and the others in the 

 neighbourhood, running through three days, I was accom- 

 panied by the Rev. H. W. Crosskey of Glasgow, who has 

 carefully studied the shelly deposits of the Clyde basin, and 

 east of Scotland : and the result of our joint observations is 

 the account that follows : The object of enlarging the list 

 of species already given (Art. 21) was quite subordinated to 

 that of working out the physical order of the beds, and 

 establishing among them definite relations. It has been 

 already remarked (Art. 21) that under the terms Boulder-Clay, 

 or Till, many distinct beds have been classed as one forma- 

 tion ; whereas there can be no doubt that if the subject were 

 properly investigated and understood, order would reign 

 among these as among the other strata of the earth's crust, 

 and that beds of a like mineral character would have also a 

 definite stratigraphical position. Not that such a series of 

 beds is to be supposed to occur universally for this is not 

 the case with the solid rock formations themselves; but 

 that if there be a considerable development of these " super- 

 ficial beds" in different places, they shall be capable of sub- 

 division into several terms, with distinct and persistent 

 characters and definite relations to one another. 



Now, in attempting to establish a physical order among 

 these beds, we are met by two difficulties which we do not 

 encounter in the case of rocky strata. The upper layers 

 under the soil are loosely aggregated and pervaded by fissures 

 which admit water, while the lower layers are retentive of 

 water. Thus a great hydrostatic pressure comes to be exer- 

 cised, and as the beds have not that continuity and coherence 

 which enables one part to lend support to another, the upper 

 beds slip down upon the lower. Such landslips also occur by 

 the undermining of a stream. In this way the order of the 



