any Evidences of former Glaciers in North Wales ? 475 



Again, an examination of the striae in hand specimens will 

 show that some of the more delicate lines diverge from the 

 general parallelism, and unite with the contiguous ones with- 

 out suffering interruption from the intervening furrows. In 

 some parts of the surface there are groups of faint transverse 

 elevations and depressions which run across many contiguous 

 striae with a gentle curve ; and in others the grooving is not 

 confined to a single plane, but seems to penetrate beneath the 

 surface, and to pervade several laminae inwards. None of 

 these appearances could have been produced by a hard body 

 sliding along its surface in right lines, as in slickensides ; and 

 I regret my inability to offer any satisfactory explanation. 

 The only conjecture I can make is, whether, as these striae 

 lie in the direction of the magnetic meridian, they may not 

 have been produced by some electric or magnetic power 

 acting upon the chemical ingredients of the bed or upon its 

 surface, and causing them to assume a polaric direction while 

 yet in a soft or semi-indurated state. They belong to a class 

 of phaenomena hitherto little noticed and still less understood, 

 but which are nevertheless of great interest and worthy of 

 especial attention, from the important bearing they have on 

 questions of physical geology, about which we are at present 

 almost totally in the dark, but which may one day throw light 

 on the original structure of our globe. 



I come now to describe a still more curious case of striated 

 surface, found on a system of rocks lying geologically far be- 

 low that just mentioned, and which, though unfossiliferous, 

 will range with the Lower Silurian formation. It occurs in 

 the bed of a little mountain stream named Clettwr, that falls 

 into the Conway a mile or two below Yspytty, having cut its 

 way through the great mounds of gravel already described in 

 that vicinity and exposed the solid rock beneath. The rock 

 is a very hard blue schist, in thick compact beds, with a dip of 

 15° N.N.E. and an obscure cleavage dipping 48° N.E., which 

 has but partially obliterated the bedding. The true stratifica- 

 tion is indicated by many thin streaked laminae which appear as 

 lines in the section, and are quite parallel with the undulated 

 surface of the bed about to be described. This surface is some- 

 what glossy and very strongly marked with a system of wavy 

 lines that preserve a general parallelism, but run into one an- 

 other, and do not long preserve their individual and separate 

 character. The surface may be more correctly said to be cor- 

 rugated and undulated than grooved, for the striated lines do 

 not possess the perfect parallelism and continuity of those near 

 Llangollen, though they maintain an uniformity of character, 

 are persistent over every part of the surface that was exposed, 



