430 RABBLES OF A GEOLOGIST. 



ney. How account for the phenomenon ? How account for 

 the three storeys, and the apportionment of the floors, like 

 those of a great city, each to its own specific class of society ? 

 Why should the first floor be occupied by Osteolepides, the 

 second by Cheiracanthi and their cogeners, and the third by 

 Coccostei ? Was the arrangement an effect of normal differ- 

 ences in the constitutions of the several families, operated 

 upon by some deleterious gas or mineral poison, which, though 

 it eventually destroyed the whole, did not so simultaneously, 

 but consecutively, the families of weakest constitution first, 

 and the strongest last 1 Or were they exterminated by some 

 disease, that seized upon the families, not at once, but in 

 succession ? Or did they visit the locality serially, as the 

 haddock now visits our coasts in spring, and the herring 

 towards the close of summer; and were then killed off, 

 whether by poison or disease, as they came? These are 

 questions which may never be conclusively answered. It is 

 well, however, to observe, as a curious geological fact, that 

 peculiar arrangement of the fossils by which they are sug- 

 gested, and to record the various instances in which it oc- 

 curs. The minerals which I remarked among the schists 

 here as most abundant are a kind of black ironstone, exceed- 

 ingly tough and hard, occurring in detached masses, and a 

 variety of bright pyrites disseminated among the darker flag- 

 stones, either as irregularly-formed, brassy -looking concretions 

 of small size, or spread out on their surfaces in thin leaf-like 

 films, that resemble, in some of the specimens, the icy foliage 

 with which a severe frost encrusts a window-pane. Still 

 further on I came upon a vein of galena ; but a miner's ex- 

 cavation in the solid rock, a little above high-water mark, 

 quite as dark and nearly as narrow as a fox-earth, showed 

 me that it had been known long before, and, as the workings 

 seemed to have been deserted for ages, known to but little 

 purpose. The crystals of ore, small and thinly scattered, are 



