Mr. Bennett on the Geology of Madeira, 393 



lava contains minute pieces of olivine ; sometimes it assumes a pris- 

 matic form, and in one place was of a moderate degree of hard- 

 ness : the principal springs of water in the island issue from this 

 stratum. On the top is the third, a greyish 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 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 grey lavas being used for the copings, &c. It v/orks 

 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 connection 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 occur in isolated 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 the deep and singular valley called the Corral^ which I had an 

 opportunity of examing for several miles, the red and grey lava alter- 

 nated five or six times. The tops of some of its barrier hills are 

 formed of columnar basalt ; here and there rising to a peak, or broken 

 into what might be termed a crystallised ridge, or tapering to a point 



3 D 



