IN THE MUD OF THE LEVANT. 67 



grains, but of particles of glassy Felspar, which 

 exhibit a number of parallel grooves, and curious 

 circular cavities.* This peculiar cancellated 

 structure of the Fel spathic fragments is even 

 still more marked in the Tufaceous layer of 

 Rio Negro. The fragments of Felspar bear no 

 resemblance whatever to the sand grains of ordi- 

 nary recent deposits. This specimen appears 

 to contain neith^ Polythalamia nor siliceous 

 organisms.f 



The other deposit which contained microscopic 

 siliceous organisms was the bone bed of Punta 

 Alta, Bahia Blanca,J in which I found a few discs 

 identical with some from St. Julian, and also a 

 few broken spicula. No calcareous organisms 

 were visible, but amongst the large grains of 

 sand was much amorphous matter, which contained 

 some Carbonate of Lime. I could, however, 

 detect no organic structure in the calcareous 

 particles. The deposit contains both shells and 

 bones. I also obtained Carbonate of Lime from 



* I am indebted to Mr. Darwin for pointing out to me that 

 these particles consist of fragments of glassy Felspar, which 

 have resulted from the long-continued attrition of erupted rocks. 



t Darwin's Journal. Second Edition. 



X Darwin's Journal, p. 83. 



