part 3] THROUGH THE ANDES OF PERL' AND BOLIVIA. 251 



Three main types o£ deposit can be recognized, succeeding one 

 another in the followino- order from west to east : — 



(1) A shallow-water facies, consisting chiefly of sandstones and shales ; the 



frequent presence of plant-remains in the latter indicates the close 

 proximity of land during their deposition. 



(2) A volcanic facies of pyroclastic origin, comprising contemporaneous 



ttiffs and agglomerates, corresponding to Forbes' s ' porphyritic 

 conglomerate ' of Bolivia. 



(3) A calcareous facies, consisting of richly-fossiliferous limestones and 



marls. 



The section described in the following pages begins at the 

 Island of San Lorenzo, which lies some 4 miles off the mainland, 

 and is separated from the promontory of La Pnnta and its 

 continuation in the shallow bank of Camotal by the narrow 

 straits of Boqueron. 



Here, and in the immediate neighbom-hood of Lima, the first or 

 shallow-water type of Mesozoic deposit attains its maximum 

 development. A detailed account of the succession has already 

 been given by Prof. C 1. Lisson,!^ to whose admirable work reference 

 may be made for a complete description of the district. 



The rocks are well exposed along the shores of the island, where 

 they are seen to consist of an alternating sequence of grey, green, 

 and red shales, olive-green quartzites and sandstones, with occa- 

 sional bands of impure limestone. The beds have an almost 

 constant dip of about 20^ south-south-westwards, and form the 

 western limb of a large anticlinal flexure, the eastern limb of which 

 is encountered on the mainland. 



A well-defined raised beach, about 10 feet above the present 

 high- water mark, is preserved along the eastern shore of the island. 



Numerous fossils have been recorded from San Lorenzo : the 

 commonest is a species of Trigonia (T. loreniii Dana), and 

 specimens of it were found in some abundance during our short 

 visit to the island, just above beach-level along the west side of the 

 first bay north of the sanitary station, the fossiliferous bed being a 

 reddish-green calcareous grit. 



This is overlain bv srrev shales and sandstone in which larsre 

 fragments of fossil wood occur. Badly-preserved fish-teeth and a 

 specimen of Hoplites (cf. Tliurmannia iliurmanni, Pictet & 

 Campiche) Avere also collected at this locality. 



In the rocks of the Caleta del Paraiso plant-remains are plentiful, 

 and from material obtained b}^ Prof. Gr. Steinmann, Dr. R. Neu- 

 mann has assigned a Wealden age to this flora. ^ 



If the beds exposed on the Island of San Lorenzo be projected 

 along their strike towards the south-east, they are seen to crop out 

 on the coast of the mainland on the south side of Chorillos Bay, 

 where, by the aid of Prof. Lisson's memoir, it was found easy to 

 follow the details of the succession and discover the fossiliferous 



^ ' Contribucion a la Geologia de Lima & sus Alrededores ' 1907. 

 ^ ' Beitrage zur Kenntnis der Kreideformation in Mittel-Peru ' Neues 

 Jahrb. Beilage-Band xxiv (1907) pp. 69-132. 



