90 GEOLOGICAL MEMOIRS. 



vestigation the fine dust which often falls during showers of volcanic 

 ashes. 



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

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

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

 formation, 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 

 infusorial iron,' &c. ; but all these long additions to ordinary expres- 

 sions are manifestly inconvenient, and the same difficulty occurs 

 with regard to the names ' tuff,' ' volcanic conglomerate,' ' pumice,* 

 ' phonolite,' &c. 



The author then suggests as a convenient nomenclature, that we 

 should denominate those minerals which do not exhibit either among 

 their actual component parts or from the conditions of their aggre- 

 gation any marks of organic existence, ' elementary tripoli,' ' ele- 

 mentary limestone,' ' elementary pumice,' &c., or together, Stcechi- 

 olite (elementary rock), in contradistinction to the other group, 

 which we may designate Biolite (organic rock). 



True Biolites, however, are not those rocks and formations which 

 simply contain fossils, but those which are deduced from and con- 



