Vol. 70.] THROUGH THE ANDES OF PEEU AXD BOLIVIA. II 



Posidonomya in these shales would appear to indicate a com- 

 paratively shallow-water facies, which is also suggested by the 

 associated red sandstones with gypsum. 



The glassy ground-mass of the rock is an indication of rapid 

 cooling, and, although the outburst appears to have been more 

 or less continuous during the building-up of the whole sheet, it was 

 doubtless of fairly long duration : thus each layer of pillows had 

 time to undergo considerable loss of temperature before being- 

 covered by succeeding layers. In many cases, however, the indi- 

 vidual pillows had not become sufficiently rigid to resist deformation 

 by the superincumbent mass. 



The absence of any marked metamorphism of the shales, except 

 the actual coating of the pillows, suggests that the shell- bearings 

 mud was in part deposited subsequently to the extrusion of the 

 lava, and gradually found its way through the interstices into its 

 present position, thus accounting for the presence of well-preserved 

 lamellibranchs, which otherwise, from their fragile nature, could 

 hardly have escaped crushing. 



Although vesicular or amygdaloidal structure is occasionally 

 noticed, it is by no means a characteristic feature of the rock, which 

 in this respect (apart from microscopical characters) appears to- 

 differ essentially from many other well-known occurrences, such 

 as that described by Mr. Clement Reicl & Mr. H. Dewey froin 

 Port Isaac in Cormvall. 1 This, however, is probably a feature due 

 less to environment and mode of origin, than to the chemical 

 composition and physical conditions of the molten lava. 



The chief points of interest about this rock, as kindly pointed 

 out to me by Dr. J. 8. Flett, are its extraordinarily fresh condition, 

 with complete absence of any trace of albitization such as occurs 

 in the true spilites, and the fact that its petrographical character 

 is distinctly of a Pacific type. 



Mr. O. H. Evans has mentioned the occurrence of pillow-lavas of 

 probably Mesozoic age in the Taltal coast-region of the Atacama 

 Desert in Chile ~ ; but I believe that they have not yet been 

 described in detail. 



With regard to the Avell-known association of radiolarian cherts 

 with pillow-lavas, no systematic search for these organisms has yet 

 been attempted in the shales of the Morro. as such detailed work 

 seems beyond the scope of the present paper. It is a subject, 

 however, which I hope to investigate later. 



Petro graph i c a 1 Description s. 



(A l & A ) :i Enstatite-andesite from the Morro de Arica r 

 Chile. (Pi. VII, fig. 3.) 



Macroscopic characters: The rock of which the pillows are 



1 Q. J. G. S. vol. lxiv (1908) p. 264. - 1 Ibid. p. 270. 



3 These index-letters refer to microscopic slides and hand-specimens from 

 the collection of igneons rocks made during the expedition, now preserved in 

 the University Museum, Oxford. 



