132 CAITHNESS FLAGSTONES. CHAP, xi 



The clay formed, layer upon layer, on the fishes, and 

 was transformed by pressure into flagstones. The pro- 

 cess of depression and elevation may have been repeat- 

 edly performed, but every elevation brought up from the 

 sea bottom dead fish without end. In fact, the commer- 

 cial value of Caithness flags consists in the amount of 

 dead fish they contain. " Thurso is built of dead fish," 

 said Eobert Dick ; " and the capitalists and labourers 

 are also maintained by the same article." 



Sir Roderick Murchison says of the flagstones of 

 Caithness, " that they are highly valuable for many uses, 

 and must prove eminently durable from the nature of 

 their composition. Their well-known durability is attri- 

 butable, in part, to the large amount of bitumen they 

 contain, which has been produced by the abundance of 

 fishes which existed at the time those rocks were depo- 

 sited, the fossil remains of which still abound. Tar and 

 gas may be distilled from them." Hugh Miller also says 

 " The animal matter of the Caithness Ichthyolites is a 

 hard, black, insoluble bitumen, which I have used more 

 than once as sealing-wax." 



But the geological formation of Caithness was still 

 in progress. These dead fishes existed long before the 

 appearance of man on the earth. If we stretch our view 

 over, long intervals, it will be found that, in consequence 

 of the depression of one portion of the earth's crust, 

 and the elevation of another, what has at one time been 

 dry land becomes covered with sea ; and what has at 

 one time been sea, at another becomes dry land; and 

 that, partly in consequence of the eccentricity of the 



