100 Prof. Hughes, Criticism of the Geological [Oct. 30, 



of the solid rock over which the icebound stones have been driven 

 or the smoothed and striated faces of the stones which have been 

 compelled to adjust themselves to the more or less restricted 

 movement of the yielding mass in which they are imbedded : the 

 harder minerals cutting the softer and the " flour of rock " pro- 

 duced by their waste acting as polishing powder on the rock sur- 

 faces. But nature has many ways of arriving at analogous results, 

 and before we proceed to enquire whether there is evidence of 

 glaciation on the fragments included in ancient conglomerates or 

 on the solid rock on which such conglomerates rest, it will be use- 

 ful to guard against cases of mistaken identity by examining as 

 many as we can of the known agents and effects of rock polishing 

 and striation and to endeavour to distinguish clearly between them. 

 With a view to this end I have arranged a series of specimens of 

 rock naturally, accidentally, or artificially, polished and striated. 

 Having regard to the difficulty of indicating on a plate those 

 minute differences upon which we have to rely in endeavouring to 

 discriminate between the various modes of smoothing or producing 

 striations with which we are concerned in this enquiry I have not 

 thought it worth while to give figures of the specimens : but they 

 are all preserved in the Woodwardian Museum with the numbers 

 by which they are referred to in this paper attached to them so 

 that they can be easily consulted. 



iii. Glacially smoothed and polished rock. 



In most cases glaciated rocks are smoothed rather than polished. 

 Inequalities are worn down and the roughness of the original 

 fracture is removed but except when wet there is rarely the 

 glistening surface to which the term polish could be applied. The 

 slates on top of a distant house after a shower flash back the sun's 

 rays but these slates are not polished. Example of such surfaces 

 is shown on Nos. 1, 2, 3. 



There are however surfaces over which the ice has forced a heavy 

 mass of stones and earthy material frozen into a solid body and 

 this when supplied with the " flour of rock " as polishing powder 

 has sometimes acted as a burnisher and left the rock with a surface 

 like porcelain, as seen on the fragment of a boulder from the 

 Clwydian Drift No. 4, or on the surface of the Jurassic limestone 

 No. 5. This lay in the path of the great glacier which came down 

 from Mont Blanc and its surrounding snowfields, crossed the Swiss 

 valley, climbed the flanks of the Jura and left a sheen on the 

 rocks that would take it, but simply smoothed the others as we 

 see on Nos. 1, 2, 3. We occasionally find that the original rough 

 surface of the rock is preserved in places while the protuberant 

 parts are polished and striated as in No. 6, from which we may 

 infer that no very large amount of abrasion has taken place on 



