412 Geological Memoirs. 



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 known 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 

 remains 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 the 

 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 



* 1st ed., p. 201 ; 2nd ed., p. 170. 



