110 DK. J. W. EVANS ON THE ROCKS OP THE [Feb. I906, 



the direction of vibration of the slower wave-front being sometimes 

 as much as 26°. It is possible that this is a secondary hornblende. 

 Porphyritic crystals of what once was olivine also occur here and 

 there. They are altered partly to a mineral of the calcite-group, 

 similar to that described below, and partly to another mineral more 

 or less linear in section, with straight extinction and a birefringence 

 rather higher than the alteration-products of the felspar above 

 mentioned ; it is not improbably phlogopite. Some serpentine may 

 also occur, but the whole forms a complex of ill-defined minerals, 

 the determination of which presents considerable difficulties. 



The most remarkable feature of the rock is the presence of a 

 number of spherical spaces up to 300 microns (0*3 mm.) in diameter. 

 These are occupied by at least two different materials in more 

 or less regular succession. Near the walls is a mass of irregular 

 spherulites ranging up to a diameter of about 20 microns. They con- 

 sist of radiating flakes of a pale-green mineral, with low birefringence 

 and parallel or almost parallel extinction. The direction of vibration 

 of the slower wave-front is parallel to the length of the sections 

 of the flakes. The refractive index is well over that of the felspar 

 and the mixture already mentioned, and lower than the monobromo- 

 naphthalene. The birefringence does not exceed 4 thousandths 

 (*004). The mineral was strongly stained with rhodamine, both after 

 drying in an air-bath and after treatment with hydrochloric acid. 

 In spite of the deficiency in the pleochroism, I believe it to be a 

 member of the chlorite group, probably the ferruginous species 

 delessite. It presents a botryoidal contour towards the interior, 

 which is filled with a transparent carbonate of the calcite-group. 

 It was attacked by a fairly-strong solution of hydrochloric acid, but 

 gave no stain with a solution of ferric chloride : this last reaction 

 indicating the absence of simple calcium-carbonate. 



Between the carbonate and the chlorite is a kind of granular 

 selvage, representing the overlapping of the two. 1 The association 

 of carbonates allied to calcite and chlorite is well known in hand- 

 specimens. Occasionally, however, ill-defined patches with higher 

 birefringence than the chlorite are seen. They may, perhaps, be 

 flakes of hydrous mica similar to those noticed in the felspar. 



These cavities appear to have been originally formed by the ex- 

 pansion of steam ; subsequently, the chlorite formed round the inner 

 surface, the centre of which was ultimately filled with carbonates. 

 They seem to be analogous to the similar structures in pyromerides. 



The groundmass of the rock presents some points of interest. 



1 It is interesting to note that, when the lower nicol is so placed as to allow 

 the vibrations of the slower wave-front of the carbonate to pass, the irregu- 

 larities of its upper surface are clearly seen, its refractive index being much 

 higher than that of the balsam or oil with which it is covered ; while, where the 

 chlorite is overlain by the transparent carbonate, the boundary between them 

 is but feebly marked, the difference between the refractive indices being 

 comparatively small. On the other hand, when the faster vibrations with 

 lower refractive index are allowed to pass, the surface of the carbonate appears 

 smooth while the variations in the surface of the chlorite are well seen. In 

 the former case, well-marked parallel fissures in the carbonate are sometimes 

 visible which are more marked than ordinary cleavage-cracks. 



