108 Frof. Hughes, Cr^iticism of the Geological [Oct. 30, 



the same time most clearly distinguishes them from glacially 

 striated rocks is the manner in which they appear to be pinched 

 to an edge and scored on both sides up to the edge, so that whereas 

 as I have already remarked (see p. 103, and specimens Nos. 22 to 27) 

 the same set of glacial striae seldom run far over the edge of any 

 glacial boulder, and flat sides are the rule ; in crushed conglome- 

 rates on the other hand the grooves follow the curvature of the 

 stone, and the edge is sometimes crushed up as if the pebble had 

 been nipped out. This is well seen in Nos. 53 and 54, and less 

 distinctly in Nos. 55 and 56. 



Whenever a rock is acted upon by pressure in such a manner 

 as to produce deformation, and there occur here and there in it 

 harder masses which do not yield to the same extent, they are 

 thrust through the surrounding matrix and are apt to be smootlied 

 or striated according to the relative hardness of the surfaces. This 

 must happen whether the included body be a pebble in a con- 

 glomerate or a concretion in a clay. The spherical or cylindrical 

 or conical bodies known as stylolites are formed in this manner and 

 their slickensided or fluted exterior is produced by the harder less 

 yielding condition of the concretionary mass, as in the examples 

 numbered 63, 64, or even by the obstruction to movement offered 

 by a shell or other foreign body as in No. 65, in both which cases 

 the surrounding matrix is dragged round and along the included 

 unyielding body. 



We must remember that many though not all of the above- 

 mentioned operations, which produce the form or smoothness or 

 polish or striation, simulating that due to glacial action, are 

 subaerial and that therefore, before such a stone could be trans- 

 ported to any situation where it could be entombed, the chances 

 are that it would have been exposed to so much wear and tear as 

 would have entirely obliterated the superficial characters it had 

 acquired. We admit therefore that it is improbable that many 

 stones polished and striated in any of these accidental manners 

 should be found in a conglomerate. But when we notice that, 

 although a large number of observers have been on the look out all 

 over the world for evidence of this kind of ancient glaciation, the 

 examjales found are generally of very rare occurrence and doubtful 

 character, we must not lose sight of those accidental modes of 

 producing polishing and strise which may be easily mistaken for 

 those which are the result of glacial action. 



viii. Supposed occurrence of glaciated stones in ancient deposits. 



Having thus considered some of the various modes in which 

 stones may have acquired both polish and striation let us examine 

 some of the cases in which it has been maintained that the 

 included fragments in a conglomerate or unconsolidated gravel 



