236 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



which have been cross-sectioned by those of more recent origin. The 

 whole appearance of this region is such as to impress one w ith the fact 

 that we have here remnants of a great ancient land-mass, that existed 

 throughout at least a considerable portion of the Palaeozoic and continued, 

 with constantly diminishing area, through Mesozoic and Tertiary times. 

 It has to-day the appearance of an extremely mountainous district, which, 

 during late Tertiary times, had become nearly, though not quite, submerged 

 and buried beneath materials belonging to that age. Its present surface 

 distribution is very imperfectly known, but whatever its area may now be, 

 it is certainly only a very small fraction of what it must have been, even 

 in early Tertiary times. I have myself observed it not only over a con- 

 siderable extent of the country to the east of the lakes mentioned above, 

 but I have seen crystalline rocks, which undoubtedly pertained to some of 

 its members, cropping out in the valley on the north side and near the 

 mouth of the Santa Cruz River and in the valley of the San Julian, while 

 the porphyries at Port Desire and at Camerones belong also to the same 

 series of more or less ancient rocks and afford a somewhat better, though 

 still quite inadequate, idea as to the former extent of this land-mass. 



The Rivers of Patagonia. 



The Rivers of the Plains. — Considering the extent of latitude and the 

 proximity of the Andes, the rivers of the Patagonian plains are remark- 

 ably few in number, and of these only three in the region under discus- 

 sion furnish a never-failing stream of water, which flows continuously 

 throughout the year from their sources to the Atlantic. These are the 

 Gallegos, the Santa Cruz and the Chico, or north fork of the latter. From 

 the mouth of the Santa Cruz River in latitude 50° to the Chubut River in 

 43°, or through seven degrees of latitude, nearly four hundred and seventy- 

 five miles, not a single flowing stream enters the Atlantic. This fact is due, 

 not so much to the extreme aridity throughout this region, as to its peculiar 

 topographic conditions, and will be more fully discussed when I come 

 to treat more in detail of the water courses of this particular district. 



Two Classes of Rivers in the Patagonia)i Plains. — From the preceding 

 paragraph it will readily appear that the rivers of these plains may be 

 divided into two classes, V\z., perpetual rivers, or those that flow continu- 

 ously throughout their entire course and during the whole year, and inter- 

 mittent rivers, or those with an interrupted flow either as to time or extent. 



