DEPARTMENT OF MARINE BIOLOGY. 193 



Kauai; Upolu has some of the essential features of Maui, though lacking the 

 incomparable Haleakala. 



Comparison of the degrees of dissection by streams shows that Tutuila is 

 the oldest of the Samoan Islands. The core of Upolu may be as old, but its 

 deeper valleys have been partly buried under newer jfloods of basaltic lava. 

 Tutuila itself was not built up by continuous eruption, for on the south shore 

 near Leone mature vallej's cut in deeply weathered (lateritized) basaltic 

 flows have been flooded with oli vine-rich basalts of almost ideal freshness. 

 Resting on these recent, broad flows, southeast of Leone, are two tuff cones 

 with wide craters now forming bays separated by Steps Point. Aunuu Island 

 is a tuff cone bearing a crater lake and is nearly of the same age. Tau, to the 

 east of Tutuila, hke Savaii on the extreme west, is young, and there stream- 

 carding has only begim. Olosega and Ofu are more lateritized, but dissection 

 is not much more advanced. Thus there is no regular increase of age in the 

 islands from west to east, as inferred by some students of the charts. 



For lack of fossil e\'idence, the geological dates of the eruptions can not be 

 given. Tutuila has lost so much substance by erosion that the bulk of its 

 lava-flows must be referred to a stage of the Tertiary period more than a 

 million years ago. The age of the younger flows on Tutuila can hardly be 

 more than a few thousand years, and it is probable that all the other Samoan 

 islands are composed of, or veneered by, lavas of post-Tertiary dates. 



Until microscopic examinations are made, no more than a general statement 

 as to the classification of the rocks collected is possible. Tutuila is chiefly 

 composed of flows, agglomerates, and tuffs of basalt, which is generally, 

 though not always, free from conspicuous olivine. With these are associated 

 flows of apparently andesitic habit and others of trachydoleritic habit. These 

 are cut by many trap dikes and by much larger bodies— necks and huge, dike- 

 like masses — of alkahne trachyte, probably true phonolite, and a type which 

 seems to be the "sj'enitic nephelite basanite" of Weber and Friedlaender. 

 The simimit of the dominating peak, Matafao (elevation, 2,141 feet) is com- 

 posed of alkaline trachj^te or phonolite filling a small elliptical neck or vent 

 through a thick mass of explosive products. Pioa (1,717 feet) is a much larger 

 body of similar rock, a monolithic volcanic neck or crater-filling. Cockscomb 

 Island is part of an intrusive phonolitic pod, about 7,000 feet long and 2,000 

 feet in maximum width. Smafler trachytic or phonohtic intrusions, either 

 necks or thick dikes, were found on Papatele and at Afono Bay. The syenitic 

 rock forms a pipe on the di\'ide between Pago Pago and Fagasa Bays. The 

 Tutuila eruptions took place in the following order: (1) basalt, andesite (?), 

 trachydolerite (?), in flows, breccias, tuffs, and dikes; (2) trachyte, phonohte, in 

 dikes and volcanic pipes; (3) ohvine-rich basalt (^Veber's "hmburgite"), flows 

 of the Leone district ; (4) basaltic tuffs and agglomerates of the lateral craters. 



Erosion has been clearly controlled by the relative strengths of these rocks. 

 The special resistance of the trachyte and phonohte to the weather explains 

 the bold projection of the peaks and ridges of Matafao, Pioa, Papatele, Afono 

 Bay, and of Cockscomb Island and the heights to the southwestward. Many 

 spurs and capes are ribbed with massive dikes of trappean basalt and other 

 rock types. The valleys have been sunk in the weaker lava-flows and the 

 still weaker pyroclastics. In other words, a notable part of the Tutuila 

 drainage is adjusted to the softer structures throughout the eastern two- 

 thirds of the island, where constructional forms have been almost completely 

 obhterated. On the other hand, in the axial region north and northwest of 

 Leone, the original lava-flow surface is still preserved as a plateau with an 

 area of more than a square mile. In that region the lavas ma}- be slightly 

 younger than those constituting the greater part of Tutuila, but the edges of 

 the plateau are dissected by valleys reaching 700 or more feet in depth. The 



