54 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



blende are to a large extent removed, and little remains bnt peroxide 

 of iron and any minute included crystals of insoluble minerals. 

 Tbe space originally occupied by the removed material may be left 

 comparatively empty, or be filled with calcite or other minerals com- 

 monly introduced into minute fissures or cavities. Augite, horn- 

 blende, and olivine, however, are often changed in a very different 

 manner, and give rise- to pseudomorphs of various kinds, probably 

 when the change was effected at a higher temperature, or, at all 

 events, less under the influence of the atmosphere. By far the most 

 important of these pseudomorphs are of a green colour, but differ 

 much in the mechanical and optical structure of the material. 



It would be tedious and occupy far too much time to consider 

 separately all the other minerals which, in a broken-up or decomposed 

 condition, to a less extent serve to give rise to the material of stra- 

 tified rocks ; those I have selected will, I trust, serve to illustrate 

 what appear to be the principal phenomena attending the forma- 

 tion of different kinds of sand, mud, and clay. It is, however, 

 very essential that we should consider the glassy base of volcanic 

 rocks. 



Volcanic Glass and Ashes in deposited Rochs. 



The general structure of obsidian, pitchstone, tachylite, and 

 similar rocks has been described by so many authors that I need 

 say little about it. Their general base is a true glass, having no 

 depolarizing action on polarized light. This property, of course, 

 serves at once to distinguish it from fragments of very many of 

 the more common minerals, but not from isotropic minerals like 

 garnet. However, in many cases the presence of bubbles, or of a 

 fluidal or vesicular structure, shows very clearly that the substance 

 under examination is a true volcanic glass. In addition to this 

 internal structure, the fragments have often a very characteristic 

 form. The melted glass may have been blown into spray, and 

 given rise to fibres and irregular spherical bulbs, as in the case of 

 PeWs hair ; or it may have been blown out by the internal evolution 

 of gas or steam into a more or less perfect pumice. The form and 

 structure of this latter are so characteristic that there is little chance 

 of confounding minute fragments of it with any thing else. 



Speaking generally, we may say that particles of pumice are made 

 up of cells with more or less curved walls of glass which occa- 

 sionally are not more than j^q qqq of an inch in thickness. When 

 broken very small, the particles may show no entire cells, but merely 

 curious irregular compound plates, due to the meeting of several 

 cells, or simply more or less curved laminae derived from cell-walls. 

 There is a gradual passage from such vesicular glassy material to 

 solid crystalline lavas ; and only an occasional fragment in a mass 

 of ash may be decidedly vesicular or contain true glass. In only a 

 few cases, like that of Pele's hair^ could we be certain that the 

 ultimate particles had been formed by a true volcanic process. In 

 some cases it would be impossible to distinguish between a volcanic 

 rock reduced to fragments during an eruption, so as to give rise to 



