8 MR. J. A. DOUGLAS 0?T GEOLOGICAL SECTIONS [vol. Ixxvi, 



fringe the coast from Ilo to Mollendo, and thence are continued 

 northwards. 



The fossiliferous coal-bearing beds of Upper Carboniferous age 

 which are found on the peninsula of Paracas, a few miles south of 

 Pisco, must also be included in this coastal zone, as distinguished 

 from the typically Mesozoic zone of the Western Cordillera. This 

 is the only known occurrence of fossiliferous Palaeozoic sediments 

 on the whole coast of Peru, and must not be confounded with the 

 statement made by A. d'Orbigny, that the beds of the Morro de 

 Arica are of Carboniferous age, since the latter have now been 

 proved to contain Jurassic ammonites. 



The section described in the following pages commences at the 

 port of Mollendo, which is situated on the crystalline rocks of the 

 coastal Cordillera. 



The earliest rock of this series is a medium-grained pink gneiss 

 (A 71), composed of irregular alternating bands of quartz, felspar, 

 and dark minerals, of which biotite is alone recognizable in a 

 hand-specimen. The rock is much contorted and fractured, and is 

 traversed by a complicated system of faults. Although there is 

 no direct evidence for its antiquity, it has the typical appearance 

 of an Archaean rock. Forming reefs and low-lying cliffs along 

 the shore as far as Mejia, it is eveiy where seen to be pierced by 

 numerous acid veins of pegmatite (A 103) and greisen (A 101), 

 the former consisting chiefly of quartz and microchne, the latter 

 of quartz and muscovite. 



Less abundant are basic intrusions in the form of dark micro- 

 crystalline dolerite-dykes (A 97), good examples of which crop out 

 on the beach at kilometre 2 and kil. 11. The relationship of the 

 rocks is clearly defined at the first locality, where a 2-foot basic 

 dyke is seen to cut both the gneiss and the minor pegmatite-veins, 

 the latter having been injected parallel to the planes of foliation. 

 The gneiss has further been penetrated by larger masses of 

 plutonic rock, both acid and basic, representing the parent bodies 

 from which the minor intrusions have been derived. 



The connexion of the basic dykes with the parent rock is, for 

 the most part, a matter of inference, suggested l>v their constant 

 association in several localities, and they appear to represent some- 

 what earlier differentiation -products; in the case of the acid veins, 

 however, the connexion is clearly visible. 



The basic rock is a fine-grained holociystalline diorite (A 70), 

 which forms large reef -like masses on the shore in the immediate 

 neighbourhood of Mollendo, where it crops out from beneath the 

 sands of the coastal pampa. Although of dark colour and basic 

 appearance, with a high percentage of ferromagnesian minerals 

 (chiefly biotite and hornblende), it contains abundant quartz. This, 

 however, in many cases appears to be of secondary origin, occurring" 

 in the form of minute veins associated with crystals of pyrites. 



(The grey granite used during the construction of the mole, and 

 frequently found washed up in blocks on the beach, does not occur 

 here in situ, being quarried farther inland near Tingo.) 



