EHRENBERG ON INFUSORIA IN IGNEOUS ROCKS. 87 



The specimens of this tuff that were examined do not merely ex- 

 hibit the organic forms distributed more or less abundantly through 

 the mass, but they seem actually made up of them, since even the 

 dust-like powder or detritus mingled with them may be considered 

 as merely fragments reduced to a very fine state of subdivision. 



An examination of the specific forms determines the singular and 

 very important fact, that the greater number of these are widely 

 spread, reaching even to Europe, and that they are of land or fresh- 

 water origin. Eleven of the number, besides these knov/n species, 

 must also be placed amongst the freshwater forms, and most of them 

 are little siliceous particles from grasses, while there is not one single 

 marine species to be met with amongst the whole number of organic 

 bodies thus composing a rock in the middle of the ocean. It yet re- 

 mains to be determined what the thickness of the bed is, and in what 

 relation it stands to the other pumice in the neighbourhood; but 

 these infusoria can hardly be considered as having relation to the 

 present scanty vegetation of the island. 



III. On a White Volcanic Rock or Tuff from Patagonia containing 

 Marine Infusoria. 



The author states, that amongst many other objects for examina- 

 tion received by him from Mr. Darwin, one of them was a white 

 tufaceous rock, described by Mr. Darwin in his 'Journal*' as cha- 

 racteristic of the tertiary formations of Patagonia, and covering a 

 great deposit, including many tertiary shells, all apparently extinct. 

 This bed (the tufaceous rock) has been incorrectly described as chalk, 

 but it much more nearly resembles a deposited felspathic mass. 



The specimen examined presented under the microscope tlie 

 character of a crumbling pumice or tuff" containing fragments of 

 infusoria. When this result was communicated to Mr. Darwin, he 

 requested the author to experiment upon the mass in a more special 

 manner, and forwarded specimens from Port St. Julian, Port Desire 

 and New Bay, accompanied by the following remarks received in a 

 letter from him : — 



" I have to thank you for your remarks on the white Patagonian 

 rock, and to state that for many reasons I had arrived at the same 

 conclusion as yourself, that it is originally a volcanic product. Un- 

 fortunately you do not mention which of the specimens of white 

 stone contain infusoria, and I think I forwarded several, with their 

 localities marked f. The formation is on a grand scale; it contains 

 much gypsum, it has the consistence of our chalk, but is perhaps 

 somewhat softer, and it has an enormous extension. At Port St. 

 Julian it cannot be less than 800 feet thick. Its average breadth is 

 at least 200 niiles, and probably more, while it extends from north 

 to south at least 550 miles." 



The author on this renewed his investigations with every possible 

 attention, and communicates the result, enumerating the species and 



* Istcd., p. 201; 2n(led.,p. 170. 



t The author had found these remains in all the specimens. 



