416 Geological Memoirs. 



tigation the fine dust which often falls during showers of volcanic- 

 ashes. 



6. The ashes which buried Pompeii were of freshwater origin, 

 arid neither sea-water or a sea-bottom seem to have had anything to 

 do with the volcanic eruption to which they were due. The forma- 

 tion, in all essential points, resembles that of Hochsimmer. 



7. From an examination of its microscopic contents it has been 

 made clear that the ossiferous beds on the Plata and at Monte 

 Hermoso, as well as those near Bahia Blanca (both of which are in 

 Patagonia), are unchanged deposits made in slightly brackish water 

 — probably the result of some great irruption of the sea upon the low 

 lands of the mainland. 



8. It also appears that the original trachyte of Mexico, the matrix 

 of the fire-opal, affords distinct indications rendering probable its 

 relation to organic forms, and suggesting closer examination. It 

 appears within the limits of possibility that all masses derived from 

 trachyte may be in a similar condition. 



9. The idea that the most minute and invisible living beings 

 exercise a mighty influence on the solid igneous framework of the 

 earth, — an idea which was at first only suggested, but is now con- 

 firmed by every fresh investigation, — renders it possible that a far 

 greater extension may yet be looked for, and we may therefore be 

 prepared to expect corresponding results on the grandest scale. 



10. The recognition of organic influence in the case of so many 

 of those rocks of which the earth's crust is made up, renders it very 

 desirable that the limits of the extent of this influence should, as far 

 as possible, be marked out. The names siliceous sinter (Kieselguhr), 

 mountain meal (Bergmehl), tripoli, polishing slate, paper-coal or dy- 

 sodil (Blatterkohle), limestone, semi-opal, hornstone, ironstone, &c, 

 now require, not indeed mineralogically but geologically, that we 

 should be able to distinguish them by some general name, so that there 

 should be no danger of describing under the same appellation matter 

 of very different kinds. This might no doubt be effected by speaking 

 of ' organic or infusorial siliceous sinter,' ' infusorial tripoli,' and 

 * altered volcanic or unaltered volcanic freshwater or infusorial tri- 

 poli ;' or we might speak of ' polythalamial limestone,' ' organic or 



