BOITLDER-CLAT OF CHESHIRE. 595 



extended throughout the whole substance that, on removal, they 

 break up entirely. 



A large proportion of Carboniferous-Limestone pebbles bear evi- 

 dence of atmospheric and chemical erosion in a variety of ways. 

 Occasionally they are weathered all over, and portions of organisms 

 stand in relief ; more frequently they are eroded in the same way 

 over a considerable surface, whilst the remainder continues intact, 

 with its ice-marks unaffected. A frequent feature is the formation 

 of channels or hollows in the blocks ; this occurs without affecting 

 other portions, which may still retain marks of glaciation. This 

 weathering of limestone appears as if caused by chemical erosion : 

 but the results so resemble those of other rocks to which this theory 

 cannot apply, that it is rendered doubtful whether it can be entirely 

 accounted for by that cause. In many instances limestone pebbles 

 have been split into fragments which are occasionally in apposition, 

 but being generally obtained from stone-heaps, they are more fre- 

 quently detached. A glaciated one found in situ in the Boulder- 

 clay is split into four fragments, which remain in apposition ; the 

 split surfaces, as well the outer portion close to them, have sub- 

 sequently been somewhat eroded since their fracture. 



It is evident that these various forms of weathering have occurred 

 subsequently to giacier-action ; and an examination of what is now 

 in progress fails to explain these peculiar phenomena. It is only in 

 Carboniferous-Limestone pebbles contained in morainic accumulations 

 that examples occur similar to, and even identical with, those 

 found in the Boulder-clay. 



Several pebbles of limestone are not only glaciated, but also 

 perforated by mollusca and sponges ; as a rule, no shells are retained 

 in the cavities. There is generally, if not always, evidence that the 

 borings have been made subsequently to glaciation. In some in- 

 stances they have afterwards also been again exposed to glacial 

 friction, and fragments have likewise been broken off prior to their 

 deposition in the clay. In two instances glaciated and perforated 

 blocks were found to be afterwards weathered, one over the per- 

 forated surface, the other as a channel-like groove on the portions 

 covered with striae. A solitary example of borings in softer lime- 

 stone (it may be of Antrim chalk) contains many shells entire, and 

 its surface is covered with SerjDulce. 



A very frequent form of weathering in stratified, slaty, and other 

 rocks is produced by their splitting into pieces, the surfaces thus 

 formed having undergone little, if any, change ; the fragments are 

 more frequently detached and separated, but sometimes are in exact 

 apposition ; or, when split into many parts, some may be contiguous, 

 whilst others belonging to the same pebble are absent. Occasionally, 

 but necessarily very rarely, pebbles have the split fragments separated 

 for a small space, having fallen from each other as they dropped 

 into the clay. In a glaciated one of Silurian sandstone found 

 at Little Eye, Hilbre (an island at the mouth of the Dee), the 

 fragments were an inch and a half from each other with their 

 relative position changed. Another, at Eock Ferrj", had the 



