GEOLOGY OF MADEIRA. 3^ 



cover 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, Varieties of 

 are not numerous. I myself saw but four, and I was in- ^^^^ ^"^ *''^'** 

 formed there are 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 remark- 

 ably fi' e 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 pumice. 

 This red lava contains minute pieces of olivine ; sometimes 

 it a-sumes a prismatic form, and in one place was of a mo- 

 derate degree of hardness; the principal springs of water in 

 the island 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 much 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 of 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 diffe^ent 

 layers. 



The fourth species of lava is of a coarse grain, is used for A distinct stra*. 

 the making of walls, and the commonest and poorest houses '"™' 

 are budt 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 mix- 

 ture of brown and red. I observed it in a stratum by itself, 

 and it did not seem to have any connection with the other 

 three kinds. 



These are the principal stratified lavas that the island af- Vansiies not 

 fords, but in the beds and rivers, particularly in that which in strata, 

 flows in the valley of the Corral, several varieties occur in iso- 

 lated masses, containing olivine and zeolite in greater or less 

 quantity, and exhibing detached portions of strata, similar 

 to those that are found in the fossa grande on the side of 

 Vesuvius, 



In 



