W. J. SoUas- striated Tr lassie Pebbles. 79 



VII. — On Striated Pebbles from the Triassio Conglomerate 



NEAR PoRTSKEWET, MoNMOUTH. 



By W. J. SoLLAs, M.A., F.R.S.E., F.G.S.; 

 Professor of Geology in University College, Bristol. 



DUEINGr a recent visit to the tunnel now in course of con- 

 struction beneath the Severn, I was taken by my friend Mr. 

 Evan Jones, of University College, to examine a heap of Triassio 

 pebbles and conglomerate, which had been exploited from the 

 bottom of one of the shafts of the tunnel on the Portskewet side of 

 the river. My attention was at once arrested by the striated and 

 polished surfaces presented by many of the pebbles, and the results 

 of the short examination I was able to make of them are embodied 

 in the following note. 



The pebbles are of all sizes, np to a foot or more in diameter, and 

 they are well rounded at the edges and corners. Though they have 

 evidently been derived from the adjacent Mountain Limestone, they 

 break with a finely granular fracture like compact grit, owing no 

 doubt to a crystalline structure superinduced by dolomitization. The 

 matrix in which they were imbedded is a dolomitic paste containino- 

 numerous grains of quartz sand; it is coloured red with iron peroxide, 

 which also colours the surface of the imbedded pebbles. 



The smoothed surface of many of the pebbles is abundantly 

 striated, especially on and around the edges and corners ; the strias 

 commence as exceedingly delicate fine lines, which frequently 

 deepen and widen in their course, till they terminate abruptly, so as 

 to present the form of a half cone ; at the deep end of the trough 

 (base of the cone) a grain of quartz sand is sometimes found im- 

 bedded. Sometimes however the striae are mere scratches thinnino- 

 out at each end; they are not always straight, but sometimes curved, 

 a whole group of parallel striee being occasionally abruptly flexed to 

 one side and then back again into their original direction. Now and 

 then a furrow as much as a quarter of an inch wide may be 

 observed, its sides being scored with delicate parallel stride. 



The presence of quartz grains in the matrix about the pebbles 

 may be connected with their striation, and when a quartz grain is 

 found imbedded at one end of a scratch, it may fairly be regarded as 

 the instrument by which the scratch was produced. But to have 

 produced the scratch it must have been (1) pressed against the 

 surface, and (2) drawn along it. That the pebbles were pressed 

 against each other, and thus exerted pressure on the sand grains Iviuo- 

 between them, is shown by the fact that some of the smaller pebbles 

 are sunk some distance into the larger, as though they had been 

 pressed into a yielding substance. Nothing but great and long-con- 

 tinued pressure could have brought about this result.' 



The cause of this pressure is to be found in the thick deposits of 



sediment which once rested upon the pebble beds. But a vertical 



pressure acting on an accumulation of loose pebbles would be 



resolved not only in directions normal to the touching-surfaces of 



1 Sorby, Cardiff Nat. See. Trans, vol. v. 



