CIIAI^ I.] BERMUDAS TO MADEIRA. 25 



lower than that of Funehal ; and the mean summer temperature 

 is 20°-67, l°-3 lower than that of Palermo, and 0°-2 lower than 

 that of Fanchal. The mean temperature of the warmest month 

 at San Miguel is 22°-6r C, and that of the coldest 12°-28 ; the 

 range between the extremes is therefore only about 10° C. 



All the islands are volcanic, and their structure recalls, in 

 every respect, that of such comparatively modern volcanic dis- 

 tricts as those of the Eifel or Auvergne. The high rugged 

 crests, which everywhere take the form of more or less com- 

 plete amphitheatres, are the walls of ancient craters, the centres 

 of earlier volcanic action. The bottom of the old crater is now 

 usually occupied by a lake, and in it, or round its edges, or out- 

 side it on its flanks, there are often minor craters, frequently 

 very pei'fect in their form, which indicate eruptions of later 

 date, efforts of the subsiding fires. The rocks, which every- 

 where stretch down in great undulating masses from the sides 

 of the craters to the sea, are lavas of different dates, some of 

 them not much more than a century old ; the wooded ravines 

 are sometimes the natural intervals between lava streams, deep- 

 ened by rivulets which have naturally followed their direction ; 

 more frequently they are valleys of erosion, worn by torrents 

 in intervening accumulations of loose scoriae ; and the splendid 

 cliffs, which form an inaccessible wall round the greater part of 

 most of the islands, show, in most instructive sections, the ba- 

 saltic, trachytic, and trachydoleritic lavas, and the rudely or 

 symmetrically stratified subaerial or submarine beds of tufa and 

 ashes, the products of successive eruptions. As a rule, soil 

 formed by the wearing-down of volcanic rocks is highly favor- 

 able to the growth of plants. It is wonderful to see how the 

 coulees of lava and the mounds of pumice and ashes, formed by 

 even the most recent eruptions — of many of which we know 

 the dates, such as those of 1512, 1(572, 1718, and 1722 — are 

 now covered with corn-fields and vineyards, and, in inaccessible 

 places, with a luxuriant native vegetation. 



