GEOLOGY AND TOPO( IK A P 1 1 V OF 1 1 A \VA 1 1 . 109 



geologic interest. Two of these are at sea level and are (illed willi walciv 

 In one the water is fresh, in the other it is salt. In many plaees the roof of the 

 caves are encrnsted with mineral deposits, sometimes several inches in thick- 

 ness. The lower eaves can only be entered at certain tides and under favor- 

 able conditions. However, they are known to be old biva conduits and evi- 

 dently extend back into the cliff for some distance. 



In several places in the yroup, but notal)ly in llaiuipepe Valley, coluimijir 

 basalt occnrs. These cnrions prisms are from ten to eighteen inches in dia- 

 meter with sides from five to seven feet in length. . They are rude six-sided 

 columns which ai)pear to be due to the peculiar contraction (tf tlic lava, usually- 

 under pressure, as it cools. 



CHAPTER X. 



ISLAND OF OAIIU. 



For obvious reasons the formation of Oahu, the metropolis of the group, 

 has received much attention from various observers, with the result that its 

 topography and geology are better known than is the case with any of the other 

 islands. 



A Laboratory in Vulcanology. 



Only a few of the more striking physiographic features of the island can be 

 referred to here, but it is a fact that on Oabn the student of natural phenomena 

 has a veritable open-air laboratory in vulcanology, stored with splendid speci- 

 mens, showing practically every phase that results from volcanic activity and 

 erosion. 



Oahu is about fifty-four miles long by twenty-three broad in i1s greatest 

 right angle dimensions. It has an area of 5.985 square miles, with a coast line 

 of 177 miles, and has its highest mountain peak 4,030 feet above the sea. In 

 outline it forms a four-sided kite-shape figure in which the foui- points miiilil 

 be said to correspond, in relative position, to the stars in the Southern t'ross. 

 Kaena, the northwest point of the island, is at the top of the cross; Makapuu, 

 the southeast point, is at the bottom. Kahnku Point, at the northeast, and 

 Barber's Point, at the southwest, correspond with the I'ight and left hand stai-s 

 in the astral figure. The shore-line of the island which connects these four main 

 points is more irregular in outline than that of any othei- island in the i:r(iui>. 

 a fact which has given to Oahu its valualile harboi- facilities. 



HUXOLL'LU HaKBOH Ph;AHL Haki'.ok. 



Beginning with Honolulu Harbor, situated at the mouth of the Xuuanu 

 stream, and about midway along the soutliern side of the island bet ween .Mal<a- 

 puu and Barber's Point, we find the most inip(>rtan1 hai-l>oi- in the Lii'nnp. It is 

 formed bv a sight indentation of the coast-line and is |)rott'cted by a coral reef 



