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C. H. HITCHGOCK — GEOLOGY OF OAHU 



It La generally supposed that the fertile red soils of Ewa plantation 

 have been derived from the decomposition and wash of these craters, 

 aided also by the ejection of volcanic ashes blown southeasterly. This 

 is not the usual direction of eolian transportation on Oahu, but the 

 eruptions might have taken place when the conditions favored such a 

 movement. There is certainly no sign of these ashes on the Waianae 

 side of the mountains. These craters of Laeloa all stand out distinct 

 from the Waianae range, and may be seen in favorable light from Ewa 

 and other localities along the railroad. They are very rarely visited. 



SALT LAKE GROUP 



The most extensive tuff region is that occupied by three craters, of 

 which the best known is Aliapakai or the Salt lake; the next Aliamau, 

 immediately contiguous, and the third an obscure, inconspicuous de- 

 pression called Makalapa. All of them together ma}' be called the Salt 

 Lake craters. The area is not less than 12 square miles, from 3 to 6 miles 

 by railroad westerly from Honolulu, and situated at the base of Koolau. 

 Aliapakai was examined by the United States Exploring Expedition in 

 1840, and was stated to contain a body of salt water nearly a mile in its 

 longer diameter and half as wide. It was only 16 inches deep. In the 

 year following the depth had decreased to 6 inches, and the whole bottom 

 was encrusted by salt firm enough to sustain a span of horses. The sur- 

 face consisted of cubical crystals in knobs and finger-shaped prolonga- 

 tions. As the lake stands at the sealevel, most authors regard the origin 

 of the saline waters as explained by the seepage of the ocean through 

 the walls or concealed crevices. In the rainy season Brigham says the 

 water is about 3 feet deep, and large quantities of water rush in from 

 the mountains through a hole near the center of the basin (a spring), 

 and the bottom is covered by a blue mud several inches deep. During 

 the dry season the lake contracts to one-third its size. The walls of 

 Aliapakai are 50 feet high on the southern border and 200 on the northern 

 side. The lowest depression is to the northwest, while there has been a 

 breaking down or removal of a large part of the rim on the east, next to 

 the government road. In the lower part of the basin on the south side 

 some of the original coral reef through which the tuff had been ejected 

 is still visible. The tuff contains large and small fragments of basalt of 

 various kinds, with chrysolite fragments up to one foot in length, and 

 there are crystals of augite, biotite, and garnet; also pieces of the coral 

 reef. The chrysolite came perhaps from the source of the chrysolitic 

 basalt described among the rock exposures at the Pali. 



To the south and west of the Salt lake one sees large boulders strewn 



