314 EAMBLES OF A GEOLOGIST. 



hardly they have fared : the rather unpleasant casualty of 

 being crushed to death must have been a greatly more com- 

 mon one in those days than in even the present age of rail- 

 ways and machinery. The reader, by passing half a bushel 

 of the common shells of our shores through a barley-mill, as 

 a preliminary operation in the process, and by next subject- 

 ing the broken fragments thus obtained to the attritive influ- 

 ence of the waves on some storm-beaten beach for a twelve- 

 month or two, as a finishing operation, may produce, when he 

 pleases, exactly such a water-worn shelly debris as mottles 

 the blue boulder-clays of Caithness. The proportion borne 

 by the fragments of one species of shell to that of all the 

 others is very extraordinary. The Cyprina islandica is still 

 by no means a rare mollusc on our Scottish shores, and may, 

 on an exposed coast, after a storm, be picked up by dozens, 

 attached to the roots of the deep-sea tangle. It is greatly 

 less abundant, however, than such shells as Purpura lapillus, 

 Mytilus edule, Cardium edule, Littorina littorea, and several 

 others ; whereas in the boulder-clay it is, in the proportion of 

 at least ten to one, more abundant than all the others put to- 

 gether. The great strength of the shell, however, may have 

 in part led to this result ; as I find that its stronger and mas- 

 eier portions, those of the umbo and hinge-joint, are ex- 

 ceedingly numerous in proportion to its slimmer and weaker 

 fragments. " The Cyprina islandica" says Dr Fleming, in 

 his " British Animals," " is the largest British bivalve shell, 

 measuring sometimes thirteen inches in circumference, and, 

 exclusively of the animal, weighing upwards of nine ounces." 

 Now, in a collection of fragments of Cyprina sent me by Mr 

 Dick, disinterred from the boulder-clay in various localities 

 in the neighbourhood of Thurso, and weighing in all about 

 four ounces, I have detected the broken remains of no fewer 

 than sixteen hinge joints. And on the same principle through 

 which the stronger fragments of Cyprina were preserved in so 



