chap. xi. Bahia Blanca. 321 



the eighth chapter. In the first section after crossing 

 the Colorado, an old tertiary formation, namely, the Eio 

 Negro sandstone (to be described in the next chapter), is 

 met with : but from the accounts given me by the Gau- 

 chos, I believe that at the mouth of the Colorado the 

 Pampean formation extends a little farther southwards. 

 Bahia Blanca. — To return to the shores of this 

 bay. At Monte Hermoso there is a good section, 

 about 100 feet in height, of four distinct strata, appear- 

 ing to the eye horizontal, but thickening a little 

 towards the NW. The uppermost bed, about twenty 

 feet in thickness, consists of obliquely laminated, soft 

 sandstone, including many pebbles of quartz, and fall- 

 ing at the surface into loose sand. The second bed, only 

 six inches thick, is a hard, dark-coloured sandstone. 

 The third bed is pale-coloured Pampean mud ; and 

 the fourth is of the same nature, but darker coloured, 

 including in its lower part horizontal layers and lines 

 of concretions of not very compact pinkish tosca-rock. 

 The bottom of the sea, I may remark, to a distance of 

 several miles from the shore, and to a depth of between 

 sixty and one hundred feet, was found by the anchors 

 to be composed of tosca-rock and reddish Pampean 

 mud. Professor Ehrenberg has examined for me speci- 

 mens of the two lower beds, and finds in them three 

 Polygastrica and six Phytolitharia. 1 Of these, only 

 one QSpongolithis Fustis f) is a marine form ; five of 

 them are identical with microscopical structures, of 



1 The following list is given in the ' Monatsberichten der konig. 

 Akad. zu Berlin,' April 1845 : — 



POLYGASTEICA. 



Fragilaria rhabdosoma. j Pinnularia ? 



Gailionella distans. j 



Phytolithaeia. 



Lithodontium Bursa. 



„ furcatum. 



Lithostylidium exesum. 



Lithostylidium rude. 



„ Serra. 



Spongolithis Fustis ? 



