28 MB. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April IOA4, 



the wide Cretaceous transgression which had taken place over the 

 older rocks, where similar beds of red sandstone are occasionally inter- 

 bedded with the limestones, I came to the conclusion that the red 

 sandstone series of Bolivia might be regarded as a shallow-water 

 facies of the fossiliferous Cretaceous limestones of the Peruvian 

 sierras. Probably it was deposited in an inland sea, cut off from 

 the clear waters of the ocean and subject to intense evaporation 

 and concentration. The presence of drifted tree-trunks in this 

 formation further suggests the proximity of land. 



After my return to England, I learnt that Prof. Steinmann had 

 visited Corqcoro, and correlated these beds with his Cretaceous 

 * Puca ' Sandstone. 



The beds, though thrown into a number of folds, have a general 

 strike to the north-north-west; where first met with, they seem to 

 dip about 25° westwards beneath the volcanic rocks of the Mauri 

 River. 



Though their outcrop is frequently concealed by the Pleistocene 

 river-gravels described below, they can be followed from Calacoto 

 southwards along the Desaguadero River past Ulloma, where they 

 dip in an easterly direction, being folded into a broad S3 T ncline 

 between this point and the village of Callapa. Everywhere the 

 beds contain abundant seams of gypsum, often of considerable 

 thickness. 



In the well-known mining district of Corocoro the sandstones 

 are richly impregnated with native copper, which occurs both 

 in the form of large irregular masses and disseminated in minute 

 fragments throughout the rock. Although a discussion as to its 

 exact mode of origin cannot be attempted here, it can hardly be 

 doubted that the presence of copper in the metallic state is due to 

 the intrusion of the dioritic rocks. 



The zone of dioritic intrusions. — The dioritic rocks appear 

 along the line of section in the imposing peak known as Comanche 

 Alto and the smaller foothill at its base, where they have been 

 extensively quarried during the construction of the railway (PL X, 

 fig. 1 & PL V). Forbes, although omitting to illustrate this line 

 of dioritic intrusions in his transverse section, compares it with 

 the ' plutonic intrusion ' of the Western Cordillera. 



In this he appears to have had little justification : for, neither in 

 petrographical characters nor in mode of occurrence, have the two 

 rocks anything in common. The western plutonic core, as described 

 above (p. 17), seems to have made its way into the overlying rocks 

 by a process of replacement rather than displacement ; while the 

 latter mode of origin is essentially the case in the present instance. 

 The sandstone beds have here been upheaved and broken through 

 by the diorite, and dip steeply away from it in every direction. The 

 igneous rock, moreover, lias penetrated them in the form of 

 numerous dykes, which are not, as a rule, well developed in the 

 ease of a batholithic intrusion. The metamorphism is not great, 

 and has resulted in little more than a hardening and bleaching of 

 the beds along the immediate contacts. 



