REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 3 



Carlsbad twins ; the two principal individuals exhibit polysynthetic striation, and their 

 extinctions are almost always unsymmetrical. The augite shows itself with the 

 characters which it assumes in basalts. The olivine sections have rounded edges, with 

 a yellowish zone of decomposition ; the interior of the mineral is serpentinised, this 

 alteration being made visible by the formation of green fibres penetrating all the 

 fissures. These sections of olivine contain a rather large number of inclusions, with 

 outlines suggesting the form of an octahedron ; these are slightly transparent with a 

 brownish tint, and are probably picotite. 



The rocks we shall now describe were found in the Canadas, a remarkable plain 

 covered with scoriae and shut in on nearly all sides by a perpendicular wall of basaltic 



Peak of Tenerife from the N.W., 40 miles. 



rock. The present terminal cone of the mountain rises from this vast plain. The 

 Canadas represent an ancient and much larger crater, in the centre of the remnant of 

 which the more modern smaller peak has been thrown up. 1 



A rock collected at the foot of the Canadas is a basaltic scoriae, the vesicles reaching 

 a diameter of 5 to 6 mm. ; it is covered with limonite, and a freshly fractured surface 

 shows a compact violet mass. Under the microscope it is seen to be very much altered ; 

 its structure is sometimes that of dolerite or that of ordinary basalt. The plagioclase 

 also shows all the transitions from rather large and twinned crystals to the microliths 

 of the paste, which appear as small striae, and in which the polysynthetic lamellae can 



1 Narr. Chall. Exp., vol. i. p. 55. 



