BASALTIC CAftONS. 179 



hundred yards in width and with quite perpendicular walls, as shown in 

 Fig. 22. The bed of the cafton was perfectly dry and whatever drain- 

 age, under ordinary conditions, takes place below the lake is subter- 

 ranean in its nature, so perfect is the dam formed by the last terminal 

 moraine. At various places in the walls of the cafton below the lake, the 

 basalts were extremely porphyritic, and at many places there were out- 

 crops of red porphyries similar to those already mentioned as occurring 

 at Port Desire on the Atlantic coast At other places were outcrops of 

 the barren Cretaceous sandstones, while in deep channels cut in the 

 eroded surface of the latter were to be seen deposits belonging to the 

 Patagonian and Santa Cruzian formations. 



After two or three days passed in this camp we moved across the val- 

 ley and, by a rather circuitous route, gained the eastern extremity of a 

 high basaltic platform, which extends eastward upon the plain from the 

 foothills of the Andes lying to the north of Lake Gio. We camped for 

 the night at some springs issuing from the base of these basalts, and 

 the following morning, having skirted their eastern extremity, we con- 

 tinued northward over the plain, which, to all appearances, offered a 

 perfectly practical highway for an almost indefinite distance. We were 

 not long in discovering our error, however, for we had travelled little 

 more than two hours in an east-by-north direction, when we encountered 

 such a labyrinth of deep and almost inaccessible caftons as rendered 

 further progress with our vehicle exceedingly difficult, if not well-nigh 

 impossible. Descending into one of the shallower of these caftons to 

 reconnoitre, I found near its head a small spring of most excellent water 

 in a well-sheltered locality, with an abundance of wood for camp pur- 

 poses and a plentiful supply of grass for our horses. At this spring, 

 which was easily accessible, I decided to establish a permanent camp and 

 explore the surrounding country. 



This camp was located near the source of one of the numerous small 

 caftons emptying into those marked Basalt Caftons on the map and lying 

 east of Lake Gio and southeast of Lake Buenos Aires. At only a short 

 distance below our tent this small cafton entered a narrow, picturesque 

 and rocky defile, some views of which are shown in Figs. 9 and 10. 

 With the exception of an occasional small spring, the bed of the cafton 

 is dry. These conditions continue for a distance of one and a half miles, 

 in which distance it receives several no less picturesque tributaries. It 



