178 mr. j. b. scrivener on thb [May 1903,. 



this point has been produced. Judging from the evidence of 

 the valleys traversed during my journey, I agree with Mr. Hatcher 

 that they have been formed by denudation. Darwin, it is true, 

 has given good reason for considering the valley of the Rio Santa 

 Cruz an old sea-strait, and Dr. Moreno says that that of the Rio 

 Gallegos is of a similar nature ; but Darwin does not say anything 

 as to the valley of the Santa Cruz being a fault-valley. Nothing 

 was seen in the valleys mentioned above to suggest such an origin ;. 

 indeed, the presence of the outlying basalt-masses — Sierra Ventana, 

 Baleria Sud, and Baleria Norte — in the valley of the Chico de Santa 

 Cruz, masses which are at the same level as the basalt on the 

 valley-wall, are direct evidence to the contrary. 



How great a part marine denudation took in the formation of 

 the valleys north of the Santa Cruz I am not prepared to say ; but 

 when we take into account the fact that morainic material has 

 blocked the head of the great Canadon Salado, and (if the evidence 

 of the Tehuelche Pebble-Bed has been read aright) the close asso- 

 ciation of the moraines, pebbles, and the former greater extent of 

 the sea, it appears at least probable that these valleys are continua- 

 tions of older valleys which existed before the sea receded ; and 

 that, since we know from evidence supplied by Darwin that the 

 recession was gradual and interrupted, the sea took a part in 

 moulding the upper terraces of the valleys over a considerable 

 portion of their lengths. Again, when, in addition to the possible 

 marine action, we consider the former greater volume of the rivers 

 and the increase in denuding powers afforded by the general up- 

 heaval of the land, the difficulty of the disproportionate size of the 

 valleys and the present rivers is reduced, if it does not vanish. 



IX. Summary. 



The Patagonian Beds are well developed on the banks of the Bio 

 Santa Cruz. They contain fresh plagioclase and hypersthene. 



The top of the Patagonian Beds is marked at Monte Leon by 

 a shell-bed and gypseous mudstone, the latter with stunted 

 Turritellce. 



A series of estuarine beds with plant-remains, and showing evi- 

 dence of oscillation of the land-surface, intervenes between the 

 marine Patagonian Beds and the terrestrial Santa Cruz Beds. 



The Santa Cruz Beds consist of mudstones and blue clay, with 

 bands of ferruginous sandstone and of Ostrea ingens-, Zittel.. 

 The mudstones contain a quantity of pumiceous material and 

 some fresh hypersthene. 



The presence of fresh hypersthene indicates that, while the Pata- 

 gonian and Santa Cruz Beds were being deposited, either 

 ejections of andesitic tuffs were taking place, or part of the 

 sediment was derived by denudation from older andesitic rocks. 



Numerous intrusions of quartz -porphyry occur in Patagonia, 

 especially in the Chubut Territory, where two bosses were 

 seen — one foliated and faulted, the other undisturbed, showing 

 that the intrusions cannot have taken place at the same date. 



