I. Bowman — Physiography of the Central Andes. 213 



across the Maritime Andes, a clear case of such antecedency. 

 The occurrence is just south of the volcano Hoailla, a full 

 half day's ride southeast of Lake Huasco. From a low, flat 

 divide, a fourth of a mile or more across, a valley begins whose 

 descent is west toward Lake Huasco, to which it is tributary. 

 The valley is at first flat-bottomed with tiny meander scallops 

 on the margin of the valley flat. It continues with this char- 

 acter a half mile or more, then deepens and narrows gradually, 

 and is finally transformed into a gorge a half mile long that 

 transects the edge of the block. 



This feature has significance in the analysis of the landscape 

 hereabout, in that it clearly establishes the fact of deforma- 

 tion after the establishment of definite drainage lines upon a 

 flatter surface. The persistence of the downcutting stream, 

 across the uplift, has resulted in the curious aspect of a stream 

 flowing westward in a direction precisely opposite to that sug- 

 gested by the general eastward slope of the block to-day. 



The conditions described in the preceding paragraphs occur 

 chiefly in the Lake Huasco region. We shall now turn to a 

 more southerly district, that of the Chacarilla mountains, for 

 topographic and structural features of the greatest importance 

 in the interpretation of the western Andes. 



Fig. 9. 



Fig. 9.— Fifteen-mile semi-diagrammatic section of the Chacarilla gorge 

 on the western slope of the Maritime Andes, east of Allianza, Chile. 



Were clear evidence lacking in other localities of a pro- 

 tracted period of erosion during which the land surface, now a 

 part of the western Andes, was reduced to a plain of slight 

 relief, it would suffice to rest the proof upon the evidence 

 afforded in the walls of the Chacarilla gorge. Here is dis- 

 played for fully fifteen miles, namely, from above the oasis of 

 Chacarilla to Algarrobal, an unconformity of exceptional defini- 

 tion. Fig. 9, which displays it diagrammatically, scarcely 

 needs interpretation. Below the unconformity is a banded 

 rock of diverse structure, only suggested by the details of the 

 figure. These structural irregularities are planed off with 

 great regularity, and are overlaid by a thick series of flat sand- 

 stones and conglomerates. The unconformity is easily located 

 in the field, not only by the structural contrasts suggested in 

 the sketch but as well by the contrasted topographic architec- 



Am. Jour. Sci.— Fourth Series, Vol. XXVEII, No. 165.— September, 1909. 

 15 



