•52 MR. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [vol. lxxvL, 



the former consisting- chiefly of orthoclase, slightly turbid through decom- 

 position ; albite and oligoclase are also present, and perthitic intergrowths 

 are not uncommon. 



Elseolite occurs as an accessory, but it has been largely decomposed. 



The coloured minerals consist chiefly of a violet-brown titaniferous augite, 

 in the form of large plates interg-rown with a deep-brown biotite and con- 

 taining abundant needles and prisms of ajjatite. 



Magnetite and pyrites are present in about equal proportions ; they are 

 frequently intergrown one with the other and surrounded by biotite. A con- 

 siderable amount of fresh olivine is also met with. 



A 124. Mica - syenite. Urahuasi. 



A dark-grey holocrystalline rock of medium grain, consisting chiefly of 

 biotite and felspar, the former in excess of the latter. 



Microscopic characters. — The felspars, which are subordinate to the 

 ferromagnesian minerals, consist chiefly of plagioclase (oligoclase-andesine), 

 but a considerable amount of orthoclase is also present. 



A reddish-brown biotite is abundant, and this is commonly intergrown with 

 yellowish-brown hornblende, the latter usually showing idiomorphic outlines. 



A small amount of colourless to pale-green augite is also present. 



Accessories include abundant apatite ; a little sphene ; brilliantly-polarizing 

 prisms of zircon ; magnetite ; pyrites ; and a green spinellid. 



A 125. Andalusite - mica-schist. Casahuiri. (PI. II, fig. 5.) 



A finely-banded biotite-schist, containing numerous large crystals of a 

 rose-pink andalusite. 



Microscopic characters. — The parallelism of the constituent minerals 

 is not well marked, and the structure as seen under the microscope approaches 

 the granulitic. 



The bulk of the rock consists of a fine quartz-mosaic, through which are 

 scattered abundant flakes of biotite and some muscovite. 



Andalusite occurs in the form of large well-defined crystals of a faint pink 

 colour, containing few inclusions. 



IV. General Summary and Conclusions. 



The foregoing paper gives an account of the geological structure 

 of the Andes of Southern Peru, as illustrated by a horizontal section 

 drawn from the port of Mollendo to the Inambari River; and a 

 comparison with a parallel section by the present writer across the 

 Cordilleras of Northern Chile and Bolivia will serve to show that 

 the main structural features there described still find expression in 

 the district under discussion. 



Several points of difference, however, will be noticed ; gaps in 

 the sequence have been filled in, whereas new gaps appear ; vast 

 areas of desert here conceal the continuation of man} 7 ' rocks that 

 are exposed farther south, while in other cases where the exposures 

 are more connected, the beds are frequently observed to undergo a 

 change in character, and additional light is thus thrown on the 

 distribution of land and sea in former geological times. 



The rugged foreshore at Mollendo differs markedly in appearance 

 from the stratified cliff's of Northern Chile. The deflexion of the 

 coast-line towards the north-west has here brought to light a zone 

 of ancient granite and gneiss, comparable with the rocks forming 

 the coastal Cordillera of the south. These rocks are shown to be 

 of typical 'alkaline' facies, and it is suggested that their origin 

 dates from a very early period antecedent to the uprise of the 



