262 RAMBLES OF A GEOLOGIST. 



largely charged with bitumen. And of a Liasic deposit of 

 this compound character, consisting in larger part of an infe- 

 rior argillaceous bed, and in lesser part of a superior one of 

 dark shale, the tile-clay of Blackpots has been formed. 



I had next to determine whether aught remained to indi- 

 cate the period of its re-formation. The tile-works at the 

 point of the promontory rest on a bed of shell-sand, composed 

 exclusively, like the sand so abundant on the western coast 

 of Scotland, of fragments of existing shells. These, however, 

 are so fresh and firm, that, though the stratum which they 

 form seems to underlie the clay at its edges, I cannot regard 

 them as older than the most modern of our ancient sea-mar- 

 gins. They formed, in all probability, in the days of the old 

 coast line, a white shelly beach, under such a precipitous front 

 of the dark clay as argillaceous deposits almost always pre- 

 sent to the undermining wear of the waves. On the reces- 

 sion of the sea, however, to its present line, the abrupt, steep 

 front, loosened by the frosts and washed by the rains, would 

 of course gradually moulder down over them into a slope ; 

 and there would thus be communicated to the shelly stratum, 

 at least at its edges, an underlying character. The true pe- 

 riod of the re-formation of the deposit was, I can have no 

 doubt, that of the boulder-clay. I observed that the septaria 

 and larger masses of shale which the bed contains, bear, on 

 roughly-polished surfaces, in the line of their larger axes, the 

 mysterious groovings and scratchings of this period, marks 

 which I have never yet known to fail in their chronological 

 evidence. It may be mentioned, too, simply as a fact, though 

 one of less value than the other, that the deposit occurs in 

 its larger development exactly where, in the average, the 

 boulder-clays also are most largely developed, a little over 

 that line where the waves for so many ages charged against 

 the coast, ere the last upheaval of the land or the recession 

 of the sea sent them back to their present margin. There 

 had probably existed to the west or north-west of the deposit, 



