474; Mr. Bowman on the question, Whether there are 



in some places so obscurely, that it would not have been per- 

 ceived to be so on casual inspection ; but in others, very clearly. 

 There was another bed lying about a foot lower in the series, 

 similarly though less distinctly striated ; and the striae had ex- 

 actly the same direction. 



It was therefore abundantly clear that these grooved striae 

 could not have been produced by glacial action, or indeed by 

 any cause operating after the original deposition or consolida- 

 tion of the beds ; inasmuch as they are nearly at right angles 

 to the course of the river, and therefore to the direction of a 

 glacier, and especially as they are found under beds in close 

 contact, that have never suffered any partial or relative dis- 

 turbance inter se*. Since, however, they exist on two sur- 

 faces in contact, and somewhat resemble those polished por- 

 tions of rocks well known by the name of Slickensides, which 

 are believed to be the effect of friction on two surfaces rubbed 

 against each other, it is worth while to examine whether they 

 might not have been produced by a similar action. As already 

 stated, the appearance of the quarry and precipice above the 

 road shows that the rock has been tilted aside en masse, and 

 has not been affected by any partial or subsequent dislocation ; 

 and the nearly horizontal position of the beds renders it ex- 

 tremely improbable, if not impossible, that they could have 

 been so rubbed against each other as to produce the furrows 

 in question. The appearance called Slickensides is found on 

 the perpendicular or slanting sides of fissures and veins, and is 

 caused by the unequal and repeated motion of the two surfaces 

 against each other. But in proportion as the direction of these 

 sides varies from the slanting to the horizontal position, this kind 

 of action diminishes, from the increasing difficulty of maintaining 

 motion on the horizontal plane. Great lateral pressure, from 

 the gravity of an upheaved mass, may suddenly and for once 

 act horizontally; but a succession of alternating throes or pa- 

 roxysms can only be propagated from below ; from any other 

 point, motion would soon be overcome by the vis inertia of 

 the mass. It is easy to understand how the upper portion of 

 a stratified rock may slide off its base when but gently in- 

 clined; but it is scarcely credible that any possible circum- 

 stances could cause it, in such a position, to be moved back- 

 wards and forwards long enough to produce the grooved ap- 

 pearance impressed upon the bed in question. 



* It is scarcely necessary to say that they differ essentially from the fur- 

 rows which traverse the mountains of Scandinavia, as described by Seff- 

 strbm (see the last Part of Taylor's Scientific Memoirs); and from the fur- 

 rowed and polished rocks of Fontainbleau, as noticed by Durocher in the 

 Phil. Mag. for Nov. 1841. 



