174 DISCOVERY REPORTS 



water which extended in a north by west to north-north-west direction for 1 5 miles, as 

 far as San Lorenzo Island off Callao, where its outer boundary was approximately 7 

 miles off the coast, and curved sharply shorewards. The southern edge was ill-defined, 

 being marked by occasional patches a few yards wide of rusty brown foam ; soon the 

 water acquired a full tawny olive and russet colour, and the foam in the wake of the 

 ship had a rusty appearance. The northern edge, on the other hand, was sharply defined 

 and lay conspicuous between the brown of the patch and the clear bluish green water 

 outside (Plate XVI, figs. 7 and 8), their respective temperatures, differing by half a degree, 

 were 16-85 ^^^ 16-34° C. Nets were towed on each side of the boundary at Sts. WS 661 

 and 662, which were 2 miles apart. In the brown patch at St. WS 662 the scum-like dis- 

 coloration seemed restricted to the surface, where bubbles and birds' feathers floated; 

 but no connection was discovered either between the surface scum and the multitudes 

 of the Peruvian cormorant known locally by the name of guanay^ which frequent these 

 parts, or with the shoals offish upon which the latter feed. The plankton seemed equally 

 abundant in the clear and the discoloured water. If the excreta of sea birds were dis- 

 charged into the sea on a large scale they might produce the oily scum, which in effect 

 bore a resemblance to a surface film of fuel oil. The Peruvian naval base lies near by, 

 but it seemed unlikely that petroleum was the cause owing to the scarcity of dead birds. 

 Although the phytoplankton catches were not large and the scum suggested no re- 

 semblance to the "yellow lenses" described bySheppard (1931) off the coast of Ecuador, 

 yet the possibility that such a mechanism may have been at work here cannot be 

 disregarded (pp. 232-3). 



Off Salaverry, coastal waters of a conspicuous olivine colour (Plate XVI, figs. 9 and 10) 

 extended 10 miles seaward: farther from land the hue changed to a sea green, and a 

 similar sequence from chalky green to blue, always graduated, was observed off Punta 

 Aguja and again off Talara. Near the Guafiape Islands irregular patches of khaki were 

 observed, but the sun set shortly afterwards and their extent was not recorded (Plate 

 XVI, fig. II). 



A patch of brilliant yellow due to a swarm of a colonial Radiolarian, probably 

 Collosphaera, was met at some 180-200 miles off Punta Aguja. It was unlike any patch 

 met close inshore, having more the appearance at a distance of the straw-like discoloration 

 of the sea described by Collingwood (1868) for Trichodesmiiim and frequently witnessed 

 in the Atlantic during the Discovery investigations. 



On July 16 at 63 miles from land in 10° 32' S, over deep water, the sea was the indigo 

 of temperate regions (Plate XVI, figs. 12 and 14) instead of the ultramarine so commonly 

 found in the open ocean in these parts, an effect almost certainly due to cloud 

 (Rayleigh, 1910). On the return voyage from Callao to Valparaiso, two sketches were 

 made in the open ocean at 255 and 32 miles from land (Plate XVI, figs. 13 and 15). 

 The former is almost identically the same hue as that figured in Plate XVI, fig. 5, 

 and is useful as a control ; the other has a deeper tint, no doubt owing to the rougher 

 sea prevailing at the time. 



^ Phalacrocorax bousainvillii, a white-breasted cormorant. 



