chap. xii. Nuevo Gulf and Port Desire. 2>73 



stratum of very fine-grained, pale brown mudstone, 

 including numerous laminse of selenite. All the strata 

 appear horizontal, but when followed by the eye for a 

 long distance, they are seen to have a small easterly 

 dip. On the surface we have the porphyritic gravel, 

 and on it sand with recent shells. 



Nuevo Gulf. — From specimens and notes given me 

 by Lieut. Stokes, it appears that the lower bed consists 

 of soft muddy sandstone, like that of S. Josef, with 

 many imperfect shells, including the Pecten Paranensis, 

 d'Orbig., casts of a Turritella and Scutella. On this 

 there are two strata of the pale brown mudstone, also, 

 like that of 8. Josef, separated by a darker coloured, 

 more argillaceous variety, including the Ostrea Pata- 

 gonica. Prof. Ehrenberg has examined this mudstone 

 for me : he finds in it three already known microscopic 

 organisms, enveloped in a fine-grained pumiceous tuff, 

 which I shall have immediately to describe in detail. 

 Specimens brought to me from the uppermost bed, 

 north of the Rio Chupat, consist of this same substance, 

 but of a whiter colour. 



Tertiary strata, such as are here described, appear 

 to extend along the whole coast between the Rio Chupat 

 and Port Desire, except where interrupted by the under- 

 lying claystone porphyry, and by some metamorphic 

 rocks ; these hard rocks, I may add, are found at in- 

 tervals over a space of about five degrees of latitude, 

 from Point Union to a point between Port S. Julian 

 and S. Cruz, and will be described in the ensuing chapter. 

 Many gigantic specimens of the Ostrea Patagonica 

 were collected in the Gulf of St. George. 



Port Desire. — A good section of the lowest fossili- 

 ferous mass, about forty feet in thickness, resting on 

 claystone porphyry, is exhibited a few miles south of 



