﻿242 



DR. A. HOLMES ON THE TERTIARY 



[vol. lxxii, 



The rock is for the greater part very fine-grained, but in places 

 small ophitic patches occur, never more than £ inch long, in w hich 

 the texture is much coarser (see PI. XX, fig. 4). On an average, 

 two of these patches may be seen in a section of half a square inch. 

 In the direction of the greatest extension the ophitic texture seems 

 to have been broken up, perhaps by flow, for porphyritic crystals 

 of labradorite and augite stream away into the fine ground-mass. 

 At right angles to this direction similar porphyritic crystals occur, 

 but less abundantly. In the ophitic patches, as usual, the felspar 

 was the first mineral to crystallize (except for a few skeleton- 

 crystals of magnetite or ilmenite which form rare inclusions). 

 The porphyritic crystals are sometimes composite, and here too 

 the felspar preceded the augite. In the ground-mass, on the 

 contrary, the augite began its crystallization before the felspar 

 (and continued beyond the latter), and the texture is minutely 

 intersertal. It is clear, therefore, that two distinct periods of 

 crystallization are represented, and that these took place under 

 conditions sufficiently different to affect not only the grain and 

 texture, but also the order of crystallization of the two chief 

 minerals. 



The felspar of each generation is labradorite, but the early 

 crystals are richer in the anorthite molecule than the later. In 

 the former, which are characterized by broad albite twinning and 

 absence of zoning, the average refractive index is 1'57. All the 

 felspars are clear and glassy, and are nearly free from inclusions 

 except around the borders of the smaller laths, where innumerable 

 dark globules associated with tiny green blebs of augite obscure 

 the edges. Only in the neighbourhood of the inclusions is there 

 any trace of alteration, the body of the felspars being perfectly 

 fresh. 



Augite also shows a slight difference in composition, according 

 to the time of its crystallization. The ophitic plates and larger 

 •crystals are grey-green, while the small granules of the ground-mass 

 frequently exhibit a brown or even a purple madder tint. 



Magnetite and ilmenite began to crystallize before the 

 other minerals of the rock, but continued beyond them. They 

 •occur in skeleton-crystals and in small irregular masses, and 

 probably the dark globules that border the felspars and fringe the 

 edges of the palagonite wedges represent the final attempts of the 

 same minerals to crystallize : for, where they occur in palagonite, 

 the colour of the latter is paler than elsewhere. 



Besides palagonite, there is a small amount of obscure inter- 

 stitial matter, some of which is colourless and feebly birefringent, 

 thus suggesting that it may be a zeolite or possibly orthoclase. 

 A green alteration -product also occurs, and may be serpentine or 

 •chlorite. 



There is nothing to suggest that palagonite has been produced 

 by the weathering of an original glass. Glass does not occur in 

 this rock ; but, in many of the vesicular basalts belonging to the 

 second stage of eruption, glass occurs abundantly, and when it 



