Sketch of the Geology of Madtira, 585 ' ■ 



otTier on tlie western side, the largest being about a Portu- 

 guese league, or fourEnglish miles, in circumference. Every 

 thing arnr.nd wears marks of having snflered the action ot 

 fire; yet 1 \\ as unable to discover any deposit of sulphur, and 

 was told that none had hitherto been found in the island. 



The varieties of strata, which I shall term generally lava, 

 are not numerous. 1 myself saw but four, and I was in- 

 formed there were no more to be met with. Three of them 

 were invariably alternating in the same order. The first or 

 lowest lava is of a compact species, containing few, if any, 

 extraneous substances, is of a blue colour, and of a re- 

 markably fine grain. Upon that, the second, which is a 

 red earthy friable lava, rests; sometimes separated by beds 

 of clay mixed with pumice, and layers of black ash and pu- 

 mice. This red lava contains minute pieces of olivine; 

 sometimes it assumes a prismatic lurm, and in one place 

 was of a moderate degree of hardness : the principal springs 

 of water in the isjand'issue from this stratum. On the top 

 is the third, a grayish lava, generally compact, though at 

 times near the surface very cellular, and containing mucli_ 

 olivine. This lava takes principally the prismatic form of 

 basalt. I have seen it in the most perfect prisms from 30 

 to 40 feet or more in height, the surface being covered with 

 scoria, ash, and pumice. These masses oi lava contain 

 more or less of what I consider to be olivine, occasionally 

 carbonate of lime and zeolite, which last assumes either a 

 crystallized or globular form, or is diffused in a thin coating 

 between the different layers. 



The fourth species of lava is of a coarse grain, is used for 

 the making of walls, and the commonest and poorest houses 

 are built of it, the blue and gray lavas being used for the 

 copings, &c. It works easier than the two other kinds 

 above mentioned, is more friable and soft, and its colour is 

 a mixture of brown and red. I observed it in a stratum by 

 itself, and it did not seem to have any connexion with the 

 other three kinds. 



These are the principal stratified lavas that the island 

 affords; but in the beds of the rivers, particularly in that 

 which flows in the valley of the Corral, several varieties oc- 

 cur in isolated masses, containing olivine and zeolite in 

 greater or less quantity, and exhibiting detached portions 

 of strata, similar to those that are found m the Fossa Grande 

 on the side of Vesuvius. 



In the deep and singular valley called the Qjrral, which 

 I had an opporiunity of examining for several miles, the red 

 and grav lava alternated five or si.x times. The tops of some 



of 



