REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 7 



prove that the section is in a zone intermediate between the preceding P : M and x : M. 

 The determination of the angles of extinction, measured from the trace of the twinning 

 of these crystals, shows that nearly all these felspars have lain parallel to each other in 

 a plane, and that the sections have been cut very nearly perpendicular to the plane of 

 symmetry, inclining slightly to the zone P : M. The following extinctions have been 

 measured for the two individuals (left and right.) : — 



Left Right 

 0° 5* 



0° 7° 



0° 13' 



These crystals of sanidine are embedded in a finely granular ground - mass con- 

 taining small lamellar felspars, which may also be considered as belonging to sanidine. 

 These microliths are arranged in tufts, and are associated with very small greenish 

 prisms belonging probably to hornblende. It is seen by the deposit of oxide of 

 iron that the basis of the rock is altered ; it was perhaps formerly of a glassy 

 nature. 



In the gulleys to the west of Fuente Pedro, a spring situated at the height of 3500 

 feet, a greyish rock speckled with prismatic crystals of sanidine was collected. This 

 rock has a plane fracture, a waxy lustre, and is very like a phonolite. Microscopic 

 examination shows that the ground-mass contains microporphyritic crystals of sanidine 

 and of plagioclase, which are almost microliths, passing into those constituting 

 the paste ; augite and magnetite appear in rather large sections, probably owing 

 their origin to the decomposition of the hornblende. The sanidine sections are 

 large, but their outlines are not sharp ; those of the plagioclases, on the other hand, 

 are well defined, notwithstanding the mechanical deformations to which they were 

 subjected. The sanidine shows the characteristic fissures of this mineral, and is 

 twinned according to the Carlsbad law. The deformations produced in this felspar 

 by mechanical action have rendered it fibrous at the extremities of the sections ; 

 many of the crystals are bent and broken. A fact that must also be ascribed to these 

 deformations is that the sections show not only undulating polarisation but also deeper 

 colours of chromatic polarisation. Where these phenomena of pressure are observed, 

 instead of seeing the usual pale blue tints of sanidine, the colour is darker ; it passes to 

 an indigo tint of marked intensity. We are led to think that this accentuation of tint 

 is caused by mechanical action, which has left its impress on all the constituent minerals 

 of this rock. The crystals of plagioclase are far more numerous than those of sanidine ; 

 they are smaller, more elongated, and bent in all directions. These deformations, 

 which are repeated in a marked way in all the sections of plagioclase, are accom- 



