174 ME. J. B. SCKIYENOE ON THE [May I903,. 



feet (Colle San Valentin 12,717 feet, and Colle San Lorenzo 12,008 

 feet), this insignificant altitude enables one better than any laboured 

 description to realize the magnitude of this rift through the 

 Cordilleras. 



I spent some weeks on the shores of Lake Buenos Aires, 

 but was never able to proceed farther than Colle Piramide on the 

 north or the Rio Jememeni on the south ; so that, although the 

 snow-peaks beyond the western limit of the lake were visible 

 through the rift on clear days, I have to rely on information 

 supplied by the Boundary Commission for the western, and, it is 

 believed, the most interesting part. 



An excellent panoramic view of the lake is obtained from the 

 high pampas above the Rio Fenix, whence the eye can command the 

 enormous basalt-plateau stretching eastward and ending abruptly 

 on the west above the gorge, part of the longitudinal depression 

 of the Cordilleras, containing the two rivers Jememeni and Los 

 Antiguos : the red foothills, furrowed with torrents ; and the vast 

 snow-fields and peaks of the Cordilleras themselves, rent by the 

 chasm which contains the central and western portion of the lake. 



Here, above the Fenix, one grasps the key of the geological 

 structure of Patagonia : the plateau, the culmination of the wind- 

 swept table-lands which rise step by step from the Atlantic coast ; 

 the longitudinal depression, formed by a series of faults cutting off 

 the plateau from the Cordilleras ; and the transverse valley, breaking 

 the continuity of the mountain-chain itself, as though it had been 

 cut with a knife. 



On descending from the pampas, a wilderness of morainic debris 

 is entered, dotted with large erratics and cut by ravines where 

 the melting snow had drained down to the Fenix. Crossing this 

 river, which, at the point where I passed, flows through a broad 

 alluvial flat perched on the steep slope between the lake and the 

 pampas, another region of moraine, sculptured (with no advantage 

 to the scenic effect) into low conical hills, is encountered, at first 

 rising some 650 feet above the bed of the Fenix, then leading down 

 by clearly-marked terraces to the swamps and dreary tracts of 

 scrub, sand, and erratics, which form the last 500 feet of the 

 descent to the lake. Continuing along the shore to the south-east, 

 a ridge formed by a pale-red, acid intrusive rock was crossed, and a 

 slightly richer area of grassy swamps entered, cut off from the lake 

 by a fringe of blown sand. These dunes are broken for a short 

 distance at the point of entrance of the Eio Fenix by the small 

 outcrop of glaciated basalt already noted, and continue southward 

 for another 2 miles, giving way then to a long stretch of morainic 

 material on the southern shore, traversed by several streams from the 

 basalt-plateau above. East of the small outcrop of basalt are muddy 

 lagunas only a few inches deep, lying on the delta of the Fenix, and 

 marshy land which rises to a ridge forming an arc of a circle round the 

 head of the lake at a distance of between 4 and 5 miles. Through this 

 ridge the Fenix has cut a steep ravine. Beyond are arid pampas, with 

 erratic boulders, rising to the Canadon Deseado, where the valley of 



