164 CAITHNESS COVERED BY SEA. CHAP. xu. 



band arose from the sea ebbing and flowing alternately 

 over the ordinary boulder clay beneath it ? And then 

 the reddish clay, so different from the clay beneath the 

 black belt. Just as if the abrading or grinding forces 

 had ceased for a time, and then set to work again." 



He was soon able,. by his unintermitting exertions, to 

 determine whether the sea had once washed over the 

 county of Caithness. 



" In these days of hasty revolutions," he says, " my 

 opinions since yesterday have changed. I am now 

 enabled to answer the question which I put to you as 

 to whether there was a sea here before the deposition 

 of the boulder clay. 



" This morning, on clearing away the clay from my 

 shell crumbs from Harpsdale, I found a piece of the 

 Common Mussel and a piece of the Eock Whelk Pur- 

 pura lapillus." 



There was no doubt about it. Not only had the sea 

 covered Caithness, but ponderous ice-rafts had gone 

 grating along the mountain valleys, grinding the rocks 

 into clay, and dropping the boulders which they contained 

 along the sea-bottom as they sailed along. Wherever 

 he went Dick found shells among the boulder clay 

 Cyprina, Venus, Turritella terebra, Mactra, and several 

 species of the genus Tellina, 



One day, towards the end of September 1848, Dick 

 went to Harpsdale, about two miles up the Thurso river. 

 " At Harpsdale," he says, " in the boulder clay, marine 

 shell fragments are to be had in abundance. / lingered 

 by this delightful section for about an hour" He speaks 



