32 ilE. J. A. DOUGLAS ON GEOLOGICAL SECTIONS [vol. lxxvi, 



remains oiPema he compares with specimens from the 'Witchellia ' 

 Beds of Cold Comfort, near Cheltenham (P. mytiloides Bronn). 1 



In a palseontological map of Pern, recently published by 

 Prof. Carlos Lissom of Lima, a considerable area lying on the north- 

 west of Puno is coloured as Lower and Middle Jurassic, from the 

 reported occurrence, at a locality known as Compuesta, of an 

 ammonite which he calls ' Heinechia anceyps? This solitary piece 

 of evidence, hoAvever, can hardly be deemed sufficient to warrant 

 the mapping of so extensive an outcrop as that depicted. Most of 

 this area is covered by transgressive Cretaceous deposits, and the 

 specimen mentioned was probably obtained from the underlying 

 Jurassic beds, which appear to be of somewhat limited extent. 



Petrographical Notes. 



(A 111) Basalt. Lagunillas. 



Microscopic characters : — This rock contains icliomorphic phenocrysts and 

 aggregates of smaller crystals of a colourless augite, showing twinning on 

 the orthopinacoid, also zonary and hour-glass structure, with less abun- 

 dant greenish-brown hornblende and brown biotite, both possessing 

 well-marked resorption-borders. 



The felspars do not occur as phenocrysts. and in this respect the rock 

 has a certain resemblance to a basic lamprophyre. 



The ferromagnesian phenocrysts are set in a fine-grained hyalopilitic 

 ground-mass, composed of felspar-inicrolites (labradorite) with a more or 

 less parallel arrangement in lines of flow, granules of augite, magnetite, 

 and corroded flakes of biotite. 



An olivine rich in iron (hyalosiderite), of a deep golden-brown colour, 

 also occurs in sporadic crystals. 



(A 112) Olivine-basalt. Saracocha. This rock occurs in the form of 

 dark microcrystalline dykes cutting the limestone series on the shores 

 of Lagunillas. It contains small phenocrysts of augite and brown 

 pseudomorphs after olivine. 

 Microscopic characters : — Augite occurs in the form of relatively-large, 

 pale-green, idiomorphic crystals, showing well-marked twinning and 

 occasional zonary structure. 



Large pseudomorphs with the characteristic outlines of olivine are also 

 abundant. These consist of a network of fibrous serpentine, crowded 

 with minute rhombs of calcite, and patches of a clear, colourless, isotropic 

 mineral. 



The felspar-crystals (basic labradorite) are confined to the ground-mass, 

 which is holocrystalline and shows little or no trace of flow- structure. 



It consists of minute, elongated, idiomorphic prisms of augite, larger 

 lath-shaped crystals of labradorite, showing lamellar twinning and zonary 

 structure, and frequent giains of magnetite ; there is also much limonite 

 in the more weathered portions, and an abundance of carbonates (calcite 

 or magnesite). 



Prom the Saracocha Lake to Santa Lucia, the structure of the 

 country is somewhat obscured by a covering of volcanic tuff ; but, 

 as w r e approach Maravillas, an extensive development of pale-grey 

 cherty limestone is again met with, being the continuation of the 

 Cretaceous limestone-series of Lagunillas. In this district, how- 

 ever, the rocks have been strongly folded, and are penetrated by 



1 In R. I. Murchison, ' Geology of Cheltenham ' 2nd ed. (1845) p. 76. 



