part 1] THROUOH THE ANDES OF PERU AND BOLIVIA. 39 



under which the rocks have consolidated. The greater freedom of move- 

 ment in the present case has hindered the ready growth of porphyritic 

 crystals. 



The phenocrysts of augite, seen in A 112, are here represented by 

 granular aggregates, which in some instances show that they originated 

 by the breaking-up of a larger crystal. 



Porphyritic flakes of biotite are not uncommon, the presence of this 

 mineral possibly being dependent on the relatively low temperature of 

 consolidation. 



An iron-rich olivine, altering to a yellowish-brown iron-oxide, is also 

 present in some abundance, with an occasional trace of the original 

 mineral still apparent. 



The ground-mass consists of granules and prisms of a faintly-coloured 

 augite, irregular grains of magnetite, and minute laths of felspar with 

 pronounced flow-structure. 



(3) The Geological Structure of the Country between 

 Tirapata and the Inambari River. 



After completing the first half of our section at Puno, we pro- 

 ceeded northwards, along the strike, to the village of Tirapata, in 

 order to gain access to the interior or ' Montana ' region. The 

 geological structure of the intervening country is for the greater 

 part concealed beneath the wide alluvial fiats, formerly covered by 

 tlie waters of Lake Titicaca. In places, however, outcrops of the 

 underlying rocks are met with, and a connexion between the two 

 halves of the section can thus be established. 



About a mile south of Puno, in a small quarry on the shores of 

 the lake, is seen an unfossiliferous grey limestone with a dip of 45° 

 south-south-westwards ; this was found to overlie a thick series of 

 red sandstones and conglomerates, which were traced along the 

 south-western shore of the lake as far as the village of Pomata. 



On proceeding northwards from Puno along the trail to Juliaca, 

 we again meet with grey dolomitic limestones, red shales, sand- 

 stones, and conglomerates, cropping out from beneath the sheet of 

 vesicular olivine-basalt of Tiquillaca, while beds of white quartzite 

 are exposed in a small ravine leading down to the railway-line. 

 Limestone again appears at the little village of Caracoto, and red 

 sandstone at the station of Juliaca. 



Thus far we have been dealing solely with lithological characters, 

 for no paheontologieal evidence was obtained from this area. 



In general appearance, however, these rocks are totally unlike 

 those in the neighbouring districts known to contain Devonian 

 fossils, and- as they bear a marked resemblance to the Upper Car- 

 boniferous or Pernio Carboniferous series, previously described from 

 the Copacabana peninsula and the district east of Comanche in 

 Bolivia, which lie along the same line of strike, they are provision- 

 ally regarded as being of the same age as the latter. 



(Prof. Lisson, in a summary of the distribution of the f ossiliferous 

 deposits of Peru, describes a small collection of Devonian fossils, 

 preserved in the museum of the School of Mines at Lima, said to 

 come from the neighbourhood of Puno ; no exact locality is recorded,. 



