GEOLOGICAL STRUCTURE OF THE REGION. 499 



The French engineer and geologist Ed. Fuchs,* in reporting on the 

 copper deposits of Boleo, on the east coast, in the vicinity of Los Tres 

 Virgines, described a gently sloping plateau made up of tuffs and con- 

 glomerates, the latter with calcareous cement and fossiliferous. The 

 fossils are Cardium, Pecten and other bivalves probably of late Miocene 

 or early Pliocene age. The deposition of these beds was preceded by 

 trachytic eruptions across the peninsula, and followed, since the eleva- 

 tion of the Boleo plateau, by the basaltic eruption which formed the 

 neighboring volcanic peaks of Los Tres Virgines. The final phases in 

 the geological history have been, according to him, the diluvial denuda- 

 tion and the carving of the modern valleys. 



THE REGION UNDER CONSIDERA TION. 



General Remarks on its geological Structure. — In the region examined by 

 the present writers the feature that first strikes the observer is the pro- 

 portionately great influence that the great diurnal changes of tempera- 

 ture have had in the disintegration and abrasion of the rocks as con- 

 trasted with the action of water which plays so important a part in other 

 regions. The gravels resulting from the disintegration of eruptive rocks 

 consist mainly of angular fragments, and each fragment is found to con- 

 sist of an aggregate of minerals scarcely at all discolored by oxidation 

 and differinor from the parent rock mainlv in their looselv coherent con- 

 dition. The steeply upturned and highly metamorphosed beds of the 

 older sedimentary rocks weather into splintery forms and thin fragments 

 of arrowhead shape, whose corners, even in the stream bottoms, are but 

 slightly worn. On the other hand, the rounding of the massive rocks, 

 such as the coarsely crystalline diorites and granites, by atmosi^heric 

 action is even more marked than in moister regions ; so that low ridges 

 just projecting above the valley level often consist of bowlder-like blocks 

 resembling a train of erratics or parts of an old moraine. The manner 

 of distribution of the great accumulations of sand and gravel in the 

 broad arroyos and larger valleys of the interior show that they have been 

 subjected to no continuous action of running water, but only to occasional 

 freshets resulting from infrequent and local rain-storms. Except on the 

 present beaches, where they have been exposed to wave action of the pres- 

 ent ocean, rounding of the water-worn pebbles and bowlders now found, 

 which are almost exclusively of massive rocks, must evidently be as- 

 signed to some earlier geological period, modern waters having only had 

 the effect of slightly concentrating material derived from the disintegra- 

 tion of the older beds. 



*Bull. Soc. Geol. de France, 3me xiv, 1886, p. 79. 



