356 Dr Jsmes Macaulay on the Physical Geography^ 



species being perhaps allied to, if not, the Alcijonium arhoreum 

 of the present time. 



The Madeira formation is, therefore, either different from 

 those with which it has been classed ; or the observations of Abel, 

 Darwin, and others, who have sought to establish their vege- 

 table origin, have been imperfect. Of this I cannot determine, 

 not having seen specimens from the other localities. The Ma- 

 deira formation, however, is an interesting one, although it be 

 different from the others, on account of the extent of the tract 

 which it covers. With regard to the shells that abound on the 

 surface of the formation, I have already quoted Mr Bowditch's 

 observations. Mr Lowe has added many to them. I have not 

 a catalogue of those determined by Mr Lowe, but remember his 

 stating that about one-fifth of them are extinct species. They 

 belong, therefore, to the upper tertiary or Pleistocene epoch of 

 Lyell. This, too, argues against the vegetable nature of the 

 formation ; for it is not likely that a wood of dicotyledonous trees 

 or shrubs (to the appeal ance of which alone the specimens bear 

 resemblance) would have existed at a time anterior to the ex- 

 tinct shells that have been deposited above them. 



This, then, seems to have been the origin of the formation. 

 A large tract of coral, of the Alcyonian tribe, has been up- 

 heaved, by the elevation of the pyrogenous rocks from sub- 

 marine volcanic action. The death and decay of the Alcyonia 

 ensued, leaving the soil covered for a great extent with a forest 

 of coral, the surface of which, from exposure to the sun and 

 elements, has been in great measure worn, and still is rapidly 

 crumbling down, forming a calcareous sand similar in compo- 

 sition to that of the specimen. Mr Darwin describes an ana- 

 logous formation (" considerably more than a mile square, 

 covered with a forest of branching coral," p. 547.) produced in 

 Reeling's Island, and by a similar cause, namely, the death of 

 the coral over a large extent, from exposure to the sun, arising 

 from change of level in the surface. There, however, the ani- 

 mal is that of the connnon coral, and the change of surface 

 was produced by a gradual change of the surface of the ocean 

 at the place. In Madeira the coral is an Alcyonium, and the 

 exposure was produced by a violent change of level from volca- 



