44:0 RAMBLES OF A GEOLOGIST. 



CHAPTEE XIII 



WE returned to Stromness along the edge of the cliffs, gra- 

 dually descending from higher to lower ranges of precipices, 

 and ever and anon detecting ichthyolite beds in the weathered 

 and partially decomposed strata. As the rock moulders into 

 an incoherent clay, the fossils which it envelopes become not 

 unfrequently wholly detached from it, so that, on a smart 

 blow dealt by the hammer, they leap out entire, resembling, 

 from the degree of compression which they exhibit, those mi- 

 mic fishes carved out of plates of ivory or of mother-of-pearl, 

 which are used as counters in some of the games of China or 

 the East Indies. The material of which they are composed, 

 a brittle jet, though better suited than the stone to resist the 

 disintegrating influences, is in most cases greatly too fragile 

 for preservation. One may, however, acquire from the frag- 

 ments a knowledge of certain minute points in the structure 

 of the ancient animals to which they belonged, respecting 

 which specimens of a more robust texture give no evidence. 

 The plates of Coccosteus sometimes spring out as unbroken 

 as when they covered the living animal, and, if the necessary 

 skill be not wanting, may be set up in their original order. 

 And I possess specimens of the head of Dipterus in which 

 the nearly circular gill-covers may be examined on both sur- 

 faces, interior and exterior, and in which the cranial portion 



