Geological Society. 463 



these hills are Penbury and Carn-Llidy, the western por- 

 tion oF the latter of which forms the promontory of St. 

 David's-hcad. 



These hills present no appearance of stratification, and 

 are composed of feldspar and horiibiend in various propor- 

 tions and states of aggregation. They are each siirrounded 

 by mantle -shaped strata of slale, elevated at a high angle, 

 and presenting the characters of graiuvakke-slate. This 

 latter is traversed by veins of quartz, from which ver\' fine 

 specimens of rock-crystal are procured. No carbon<ite of 

 Imie appears to be contained either in the unstralified trap 

 or in the slaty grauwakke; nor did there occur in them, 

 with the exception of one equivocal instance, the smallest 

 trace of any organic remain. 



May 15. The president in the chair. — An account of 

 the island of Teneriffe, bv the hon. Henry Grey Bennct, 

 (member of the Geological Society) was read. 



The greatest length of this island from north to south is 

 about seventy miles, its greatest breadth does not exceed 

 thirty miles. In the south-western part of the island is si- 

 tuated the mountain called by the Spaniards El Pico di 

 Tiede, but better known by the name of The Peak of Te- 

 neriffe, the height of which, from the mean of several ob- 

 servations, appears to be about 12,500 English feet. 



The rocks and strata of this island appear to be, wholly 

 volcanic. A long chain of mountains passes through the 

 interior, sloping on the eastern, western, and northern 

 sides to the sea, but on the S. and SW. elevated into 

 nearly perpendicular mountains which are intersected by 

 deep and narrow ravines. 



The lowest bed of the island is porphyritic lava composed 

 of hornblend and feldspar, in its upper part porous, sco- 

 riform, and sometimes passing into the state of pumice. 

 Upon this rests a bed of the same substances as already 

 mentioned, but in structure nearly approaching to green- 

 stone. This is covered by a thick bed of pumice, which 

 itself is overspread with basaltic lava, on which in many 

 places rest beds of tufa and volcanic ash. 



This basaltic lava decomposes sooner than any of the 

 other rocks, and contains the greatest variety of imbedded 

 substances: it is sometimes divided by a layer of olivme 

 in crystals some inches long, and is often intersected by 

 thick veins of porphyritic slate. Zeolite and chalcedony 

 also occur in it. 



The number of small craters and extinct volcanos is pro- 

 digious : — ihey are to be found in all parts (jf the island, 

 G g 4 but 



