172 Geological Society. 



lime. Where the soils have been overflowed with lava, the vege- 

 table remains are charred, and the soils have been burnt to the co- 

 lour and hardness of brick ; and if the overlying lava is of great 

 thickness, a columnar structure appears. This conversion of soil 

 into brick has been observed in the Azores and elsewhere. 



The principal chain of mountains must at one time, Mr. Smith 

 states, have been much higher, because their very summits consist of 

 beds which are met with only at the base of active volcanic cones. 

 There is consequently no great crater in the island, but there are 

 the ruins of several truncated craters, and many small lateral cones. 

 The most extensive of the former is the Curral dos Freiras, an im- 

 mense ravine about three miles in length, one in breadth, and 2000 

 feet in depth, and open on its southern side. The beds of basalt, 

 tufa and ashes of which it is composed, dip outwards to the base of 

 the mountain, and parallel to its surface. Mr. Smith is convinced, 

 the volcanic products of the island being subaerial, that this is not 

 a crater of elevation, though it agrees with the characters which 

 have been assigned to such craters ; and he is further induced to 

 infer, from the resemblance of the Curral dos Freiras to the more 

 ancient portions of Teneriffe and the other Canary Islands, said to 

 be craters of elevation raised from beneath the level of the sea, that 

 a wrong conclusion has been drawn respecting them. He does not 

 object to there being elsewhere true craters of elevation. 



The principal lateral cones are to the west of Funchal, but they 

 are in general so completely blotted with vegetation that their struc- 

 ture is concealed. In the ditch of a fort constructed on one of them, 

 called Pico de St. Joao, a scoriaceous conglomerate intersected by 

 minute basaltic veins is exposed ; and a similar conglomerate occurs 

 in the fortified island in Funchal Bay, also at the eminence at the 

 landing-place. Some of these cones are covered by beds of lava 

 and tufa erupted from craters at Cape Giram, in one instance to the 

 thickness of 1400 feet. The beauty and regularity, within limited 

 distances, of these volcanic strata, and the richness and variety of 

 their colouring, are exceedingly striking. The most remarkable vol- 

 canic series, amounting to many hundred beds, is at Cape Giram, the 

 cliff, 1 600 feet in height, being stratified from the base to the summit. 

 It has been rent in many places, and the fissures which terminate up- 

 wards in acute angles, have been filled with lava ejected from below. 

 Non-volcanic Rocks. 



The limestone of San Vincente, Bowditch considered to be trans- 

 ition, but Mr. Smith shows that it belongs to the tertiary epoch, 

 yet he believes it to be the fundamental rock of the island. It 

 crosses a mountain stream between 2000 and 3000 feet above the 

 level of the sea, and abounds in zoophytes and marine testacea be- 

 longing to the genera Cardium, Pecten, Pectunculus, Spondylus, Cy- 

 prcea, Valuta, Fasciolaria, Strombus and Murex. The state of preser- 

 vation, generally in that of casts, rendered it impossible to determine 

 accurately the species. The limestone is traversed by two dykes of 

 basalt, and it lies immediately under the Paul de Serra, a volcanic 

 plateau which rises 2500 feet above the limestone. 



