EHRENBERG ON INFUSORIA IN IGNEOUS ROCKS. 85 



scopic bodies recognised as forming an integral part of the Rhe- 

 nisli volcanic tuff; and of these seventy-two are Polygastrica and 

 twenty-two Phytolithariay and all without exception consist of land 

 or freshwater forms. Only four or five of the whole number are 

 unknown and peculiar. 



7. With regard to the particular species, the author remarks that 

 the toothed Eunotice, viz. Eunotia triodon (having three teeth) and 

 E, diadema (with six teeth), have not hitherto been met with by him 

 in a living state in Germany, and are generally considered as north- 

 ern fossil forms in Sweden, Finland and North America. The first of 

 them was however found living near Salzburg by the late Dr. Wer- 

 neck, and has recently been recognised in atmospheric dust from the 

 Cape de Verd Islands. There are also three species of Bihlaria 

 not now known to exist in Europe. 



8. Among those whose remains form great masses in the Rhine 

 district may be enumerated Discoplea comta and Pinnularia viri- 

 dula. The first is only at present known as an existing species on 

 the high lands of Koordistan, but the other is common in a living 

 state everywhere in the vicinity. The Discoplea occurs also in the 

 phonolitic incrustation of Hochsimmer and at Wistershan in Bohe- 

 mia, and is also present in the ashes which have buried the ancient 

 city of Pompeii ; and a very similar species is found in the tertiary 

 tripoli of Virginia : it is closely allied to Gaillonella crenulata. 



The author mentions that he has seen and carefully avoided errors 

 of observation arising from the resemblance of some forms of infu- 

 sorial cases to fragments of crystalline bodies, such as sodalite and 

 leucite, which are often present, and might readily be the cause of 

 mistaken descriptions. 



9. If, after a further extension of our knowledge as to the circum- 

 stances of the case at Hochsimmer, it should appear possible that 

 this deposit might have been formed by a shower of ashes, or by 

 projectiles consisting of a fine dust fused together into a mass with- 

 out the presence of moisture, such showers occurring at intervals and 

 after the lapse of short periods of time ; or if the local conditions 

 should require that such ashes have been driven by a steady wind 

 into a dry crater-shaped hollow, where they have been received and 

 accumulated in layers, such a mode of accounting for the phoenomena 

 would best agree with the result of the author's investigations, and 

 his knowledge of the materials of which the deposit is made up. 



10. With regard to the very puzzling question, whence these infu- 

 soria and Phytolitharia came, he suggests, partly on account of their 

 including forms not now met with in a living state, that they may 

 possibly have formed layers of turf, or perhaps brown-coal, which 

 by some accident have come within the range of volcanic activity ; 

 and owing to their incombustible and almost infusible condition 

 have been erupted as showers of ashes. Such layers of infusoria he 

 has already shown to exist, forming and accompanying the fissile 

 coal near Siegburg and Geistingen on the Rhine, althougii on the 

 Upper Rhine none have yet been determined. The tuff and firestojie 

 appear to have had a similar volcanic origin, but to be the result of 

 eruptions of mud in which the infusorial strata have not been repeat^ 



