Professor Naumann on Smoothed Rock Surfaces. 163 



as are ground on more than one side, tlie occurrence of 

 smoothed surfaces on the walls of narrow clefts, and, finally, 

 also, by the absence of all foreign rolled stones and blocks 

 on the porphyritic hills. Now, since masses of sand and clay 

 cannot be supposed to have moved forward of themselves, 

 we require the assumption of some transporting agent. 



(2.) The grinding material must have been borne along 

 the rocks under a heavy pressure. Without pressure, the 

 grinding action is inconceivable. This pressure could not 

 have been due to the grains of sand themselves ; at least, 

 the smoothing of vertical or overhanging cliffs, as it occurs 

 here, cannot possibly be explained on such a supposition. 



(3.) The moving force can have operated only slowly, and 

 must therefore have continued in action during a considerable 

 time. 



(4.) The moving force must have been regular, and have 

 acted constantly in the san;e direction. 



(5.) The vehicle of the grinding material cannot have been 

 water. It would have been impossible for water to have cut 

 out parallel groves by means of sand. Above all, the idea of 

 violent and sudden cataclysms must be excluded. 



(6.) The vehicle of the grinding material must have been a 

 firm mass, but yet must have possessed the character of plas- 

 ticity in some degree, however slight. That it was not a fluid 

 mass, but a firm and stiflP one, is evident from the following 

 considerations ; namely, that no substance, bearing the grind- 

 ing material, except a firm and stiff mass, could have exer- 

 cised the requisite pressure ; and that it is only stiff masses 

 which could have been driven forwards in directions sloping 

 upwards, such as are exhibited locally by inclined grooves, the 

 inclination of these sometimes amounting even to 20 degrees. 

 But that the mass was also to a certain extent plastic, that is, 

 yielding, or susceptible of change of form, may be inferred from 

 the following additional considerations ; namely, that the 

 smoothing is continued over all the smaller inequalities of the 

 surfaces, that it frequently sinks down into depressions, and 

 rises over prominences, without being thus in any consider- 

 able degree disturbed, and that even rents and deep clefts in 



