134 GEOLOGY. [chap. vi. 



plants. The fossil bed, as it is called, of Canical, forms no 

 exception to this statement. On that neck of land the wind 

 blows away the basaltic sand from many fantastic forms of 

 calcareous infiltration, which present the aspect of roots 

 turned into limestone. These pseudo-roots, however, showing 

 no other structure than that of an aggregation of lime and 

 sand, probably owe their forms to the direction of the roots 

 of plants which may have grown on the spot at no very dis- 

 tant time, but at a time when a portion of calcareous tufa, 

 since weathered away, supported a vegetation which no longer 

 exists. In the same place is a deposit of land and fresh- 

 water shells. Most, if not all of them, have been identified 

 with species existing either in Madeira or in Porto Santo. 



All the volcanic beds, then, of which Madeira consists, 

 whether tufa, conglomerates, or lava, correspond with those 

 which in the Mediterranean and other parts of the world 

 appear to have been upheaved from the bed of the sea by a 

 great and general explosion of subterranean forces at the 

 Miocene period of the tertiary epoch. There is no crater of 

 eruption to be found from which lavas can be presumed to 

 have run down, or the tufas to have been ejected. The whole 

 island, from the lowest of the lateral knolls to the highest 

 peaks of the mountains, consists of tufas and conglomerates 

 piled upon each other, composed of trachytic and basaltic 

 materials, cemented by a similar paste, and injected by 

 basaltic dikes which rise from below and penetrate them, 

 sometimes partially, and sometimes to their very summits. 

 There are points, as, for instance, in the vast detached hill 

 of the Penha d'Aguia, where it is easy to observe that these 

 dikes spring from beds which appear as a vesicular scoriaceous 

 lava at the waters edge. It is evident, therefore, that at the 

 time when the elevation of the mountains took place, the 



