46 THE VOYAGE OF H.M.S. CHALLENGER. 



sections showing these striae have almost always undulated extinction, and are grooved 

 by fissures ; they are sometimes broken in several pieces and cemented together by the 

 matrix. These facts clearly show that the felspars exhibiting those striae have been 

 subjected to mechanical strain, which has induced a more or less pronounced lamellated 

 structure, and this, under the microscope, has an appearance resembling that of a plagio- 

 clase. These sections, then, are nothing else than sanidine modified by mechanical 

 action. But there is another kind of alteration in this felspar to which attention must 

 be drawn. We have said above that the large sections of sanidine almost always appear 

 corroded at the edges. This action of the magma is not confined to the border of the 

 crystal, but in some cases has affected the whole mass, softening it and transforming it 

 almost beyond recognition. The facts, as they were observed in a great number of 

 specimens of trachyte from Ascension, were as follows : — Certain sections, which were 

 naturally supposed to be ground-mass, so crammed were they with microliths of 

 irregular outline, extinguished polarised light as if they formed one crystalline indi- 

 vidual. "What seems to go against this interpretation is, that these sections are filled 

 with the same felspathic microliths composing the ground-mass. On examining them 

 more closely, however, one can find all stages of transition represented, from the perfect 

 crystal of sanidine downwards, and the conclusion must be that they are nothing else 

 than large crystals of sanidine attacked by the action of the magma. In fact, some of 

 these patches, with half-effaced contours, are surrounded by little crystals of sanidine 

 forming an external zone, and encroaching on the primitive crystal, sometimes to the 

 centre. The magma, in which these crystals of sanidine floated, may be admitted to 

 have penetrated them in some way, and to have given rise to the microliths. The 

 influence of the fused mass had not been sufficient to make the crystals lose their indi- 

 viduality completely ; their outlines were effaced, and they were invaded by microliths, 

 but they did not mix entirely with the magma, and hence did not lose their molecular 

 structure. 



Augite is the second essential element of these trachytes. We have said that it 

 never attains the dimensions of the sanidine ; it always occurs in the prismatic, almost 

 acicular, form, and is confined in the ground-mass with the little lamellae of sanidine. 

 In most cases augite is associated with a vitreous base. The small crystals are 

 greenish, slightly dichroic ; the octagonal form of sections perpendicular to the vertical 

 axis is rarely seen ; the angle of extinction is often greater than 45°. The microliths of 

 augite are sometimes reduced to mere lines, especially in those specimens where the 

 vitreous matter predominates. These fine needles are nearly always altered, as can be 

 seen from the yellow tint they assume, the colour changing from green to yellow or 

 brownish red. In some rather rare cases they become fibrous, as if they had been 

 subjected to uralitisation. Sometimes microliths belonging to a second generation are 

 observed ; the comparatively large crystals are surrounded by an outer zone of 



