518 



LAVAS OF MADEIRA. 



[Ch. XXIX. 



Madeira, although some of the newest currents, hke those at the foot of 

 the cones, m, n, o, fig. 653, are basaltic. 



I may hei'e alhide to another feature in the mineralogical structure of 

 Madeira, namely, that most commonly the uppermost of all the volcanic 

 rocks, when we ascend to heights of 1200 feet or more above the sea, 

 consist of compact felspathic trap, with much olivine, separating into 

 spheroidal masses several feet in diameter, especially when some of the 

 contained iron has become more highly oxidated in the atmosphere. M. 

 Delesse, after examining my specimens, informs me that in France they 

 would call this rock basalt, although it is often without augite, and 

 simply a mixture of blackish green felspar with olivine. Whatever name 

 we assign to it, the superficial envelope of the island, not only in the line 

 of section followed in fig. 653, p. 513, but also very generally, may be 

 said to consist of this trap, except near the sea, where basalts occur which 

 have not the same spheroidal structure. 



Among other indications of a considerable difference of age, e' ^u in 

 the superficial volcanic formations of Madeira, I may remark, that many 

 of the central peaks, such as a, fig. 653, seem to be the mere skeletons 

 of cones of eruption ; whereas the forms of the more modern cones, such 

 as M, N, 0, are regular, and have no protruding dikes on their summits or 

 flanks. 



The newest lavas also in Madeira have, in one district at least, a 

 singularly recent aspect as compared to those of older date, which are 

 decomposed superficially, often to the depth of several feet or yards. I 

 allude to the lava currents near Port Moniz, one of which is as rough 

 and bristhng as are some streams before alluded to in Palma (p. 508) of 

 historical date. I am indebted to Mr. Hartung for the annexed drawing 

 of a lava at Port Moniz, which I did not visit myself. It is traversed by 



Fig. 656. 



Surface of lava near Port Moniz, K W. point of Madeira; from a drawing by M. Hartung. 

 a. Channel traversing the lava. 



a channel, a, like one of those already described, p. 503. For how long 

 a period such characters ;iiay be retained is uncertain, so much does this 

 depend on the mineral composition of the rock. Some of the lavas of 

 Auvcrgne of prehistorical date and certainly of high antiquity, are almost 

 as rugged ; so that this freshness of aspect is only a probable indication 

 of a relatively modern origin. 



