254 MR. J. A. DOUGLAS O^ GEOLOGICAL SECTIONS [vol. Ixxvii, 



Three distinct forms may be noted : 



Form a. Anterior ribs very regular, straight or slightly concaTe towards 

 the umbo, forming an angle of 45^ with those of the posterior series, and 

 cutting the growth-lines at an angle of about 30°. 



Form h. Anterior ribs almost parallel to the growth-lines, slightly concave 

 towards the umbo near the anterior margin ; but midway between the 

 margin and the angle of the V this gives place to a distinct convex 

 curvature. Angle of V's = about 80°. 



Form c. The convex curvature noted above becomes a distinct re-entrant 

 angle, separating a series of obtuse anterior Y's from an acute posterior 

 series. 



The above-mentioned beds of the Island of San Lorenzo, Salto 

 del Fraile, and the Morro Solar form part of the western limb of a 

 large anticlinal Hexure, and the corresponding members of the 

 eastern limb are met with in the coimtry north and north-west of 

 Lima, where exposures are nmnerous and fossils are common at 

 more thanj^ne locality. 



The axis of the anticline appears to run in an ajDproximately 

 north-and-south direction midway between Lima and Callao, for 

 black shales and quartzites, much veined w4th calcite, are seen dip- 

 ping 35° east-north-eastwards immediately north of the city and 

 again at the Cerro Palao ; Avhile at Puente Inga, west of this line, 

 and again farther north at Ancon, the beds dip in the opposite 

 direction. 



At the last-named locality the rocks consist of unfossiliferous 

 quartzites. similar in character to those of Salto del Fraile, with a 

 thick interbedded sheet of andesitic lava. The latter seems to be 

 of contemporaneous origin and not to partake of the nature of an 

 intrusive sill, for it is associated with beds of porphyritic breccia 

 containing angular fragments of a similar rock. It presents the 

 appearance of being made up of a number of large spheroidal 

 masses, but it is difficult to sa}^ whether this is true pillow-struc- 

 ture or merely a phenomenon due to weathering along joint-planes. 

 Similar rocks can be followed along the coast towards Huacho. 



One of the most important fossiliferous localities is that known 

 as Puente Inga, a bridge over the Rio Chillon, some 10 miles north- 

 west of Lima. The rock consists here of a peculiar and very 

 characteristic pale-green and grey laminated clay, with a soft 

 unctuous feel. Fossils are abundant, ammonites being the chief 

 forms ; but these as a rule are much crushed and badly-preserved, 

 often merely in the form of casts. 



If we judge from the westward dip of the beds and their position 

 near the core of the anticline, it would ap^^ear that these belong to 

 the oldest formation in the district and underlie the dejDOsits of 

 San Lorenzo. It must be remembered, however, that a large tract 

 of the intervening countrj^ lies below sea-level, wherefore the actual 

 continuity of the sequence cannot be proved ; and, although no 

 counter-dips are to be observed between this point and the coast, a 

 marked deflection of the strike is noticed when the beds are again 

 encountered on the island. 



