262 Mil. J. A. DOUGLAS OlST dEOLOGICAL SECTIOIS^S [vol. IxXTli, 



thickness of overlying rock, and appears to have been injected into 

 a series of Cretaceous deposits in the form of a huge laccolite, half 

 of which only is now preserved. 



A further phenomenon of deep-seated intrusion is also to be 

 seen in the centre of the valley, immediately south of the railway 

 at Kilometre 5 (from Ticlio), where, as shown in the accompan^dng 

 sketch-map (fig. 2, p. 260) the dacite is itself penetrated by an 

 irregular mass of coarse-grained, granitoid, plutonic rock, having 

 the composition of an acid granodiorite. Although hand-specimens 

 were collected showing the junction of the two rocks, the actual 

 margin of contact is not sha]-ply defined : the chief modification 

 being a slightly-increased development of biotite in the grano- 

 diorite, where dacite-material has been assimilated. This, however, 

 is by no means a constant feature. 



In no instance Avas an included xenolith of the earlier rock 

 encountered, and it therefore seems probable that the central or 

 deeper-seated portions of the laccolite were still in a pasty or semi- 

 molten condition when the plutonic invasion took place. Along 

 the margins, however, consolidation was evidently already complete, 

 for the dacite is here pierced by sharpljr-defined dykes of quartz- 

 porphyry given of£ from the main mass, and differing from it only 

 in texture and in the presence of porphja'itic constituents. One of 

 these dykes can be traced towards the head of the valley cutting 

 across the railway, where the latter ascends in zigzag fashion. A 

 second passes through the divide bounding the valley on the south, 

 and crops out as a vertical wall on the side of the succeeding 

 valley of Buenaventura. 



The above-mentioned granodiorite is but a portion of a much 

 larger mass which is met with farther down the valley at the foot 

 of the cliffs below the lake of Morococha. The difference of level 

 between the two exposures amounts to over 1000 feet ; but this 

 can be readily explained by assuming, as was doubtless the case, 

 that the superficial portion of the deep-seated magma followed the 

 line of weakness previously taken by the laccolitic intrusion, of 

 which it was probably the parent body, and thus made its way 

 much nearer to the surface. 



The presence of plutonic rock at no very great depth below the 

 present surface of the country, as indicated in the section here 

 described, is based on the above-recorded facts. 



We must now return to our main line of section. This attains 

 its highest elevation near Ticlio, and is thence continued eastwards 

 down the valley of the Yauli, a tributary of the Mantarp, which it 

 enters just before Oroya. The country is here seen to differ vastly 

 in character from that on the west of the Avatershed. 



The relatively Avide, flat-bottomed valleys, to a great extent 

 choked Avith morainic material, which form so characteristic a 

 feature of this high-level region, present a marked contrast Avith 

 the deeply-cut precipitous gorges of the Pacific slopes ; and rocks 

 of volcanic origin, which up to this point have been the dominant 



