438 Mr. Alison on the Geology of Tenenffi, 



milar situations to the first; the colour is yellow and hard, 

 but it is brittle, and rough to the touch. The third sort, called 

 by the inhabitants Tosca, is a species of piperino ; the colour is 

 a dirty white, and occurs in beds varying in thickness from 

 twelve inches to twelve feet: it is only a carbonate of lime 

 mixed with white rapilli. It is sufficiently tenacious for build- 

 ing purposes, and is used for the upper part of walls and ter- 

 races. The fourth variety is of a dark-gray colour, or almost 

 approaching to a brown, and is used by the inhabitants for the 

 formation of filtering stones. At Gandelaria, about eight miles 

 from Santa Cruz, it is found in great abundance, and superior 

 to any stones for the same purpose that I have seen in Eng- 

 land. The price being only a dollar or a dollar and a half 

 for one of large dimensions, it is an appendage to every house ; 

 the form of them is a rectangular parallelopiped surmounting 

 a hemisphere, whose diameter is equal to the side of the square 

 which serves for the base ; the} 7 are placed in a wooden frame 

 surrounded by lattice-work to keep the water cool which is 

 filtering. 



Two or three varieties of obsidian are to be observed in 

 Teneriffe. The first is in immense blocks weighing from forty 

 to one hundred tons ; they are to be found in the Canadas 

 near the foot of the Peak, at an elevation of 8100 feet above 

 the sea, and appear to have been thrown out by the last 

 eruptions ; they generally approach a spherical form, and 

 many of them are split or broken into fragments by their fall. 

 The colour is a greenish-black, with a chatoyant lustre; the 

 fracture is irregular or hackly, and separates by a slight blow : 

 it frequently presents on the outside a fibrous appearance, 

 denoting its passage into pumice, and it generally contains 

 large white crystals of semi-vitrified felspar. The second 

 variety is of a very opposite character : the colour is a jet 

 black, possessing internally a shining vitreous lustre; it is 

 hard and compact, breaking with a conchoidal fracture, and 

 is translucent at the edges. The inhabitants call it Tobo?ia, 

 which was the name given it by the Guanches or ancient in- 

 habitants, who formed all their cutting instruments of it. It 

 is sometimes found in streams, which in cooling have formed 

 large detached blocks, and in other situations it appears in 

 long continued currents : there is one of considerable thick- 

 ness extending from the north side of the Peak to the district 

 of La Guancha, in the valley of Icod, a distance of nine or 

 ten miles. The third variety has all the qualities of the se- 

 cond, but it is of a dark-green colour. I have only found it 

 in small pieces. 



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