GEOLOGY AND TOPOGRAPHY OF HAWAII. 109 



geologic interest. Two of these are at sea level and are filled witli water. 

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

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

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

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

 dently extend back into the cliff for some distance. 



In several places in the group, but notably in Ilanapepe Valley, coluiiiiiar 

 basalt occurs. These curious 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 appear to be due to the peculiar contraction of the lava, usually 

 under pressure, as it cools. 



CHAPTER X. 



ISLAND OF OAIir. 



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 ease with any of the other 

 islands. 



A LaBOR.VTORY IX VULC.\N0L0GY. 



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

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

 has a veritable open-air laboratory in vuleanology, 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 its 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 four points might 

 be said to correspond, in relative position, to the stars in the Southern Cross. 

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

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

 Barber's Point, at the southwest, correspond with the right and left hand stars 

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

 points is more irregular in outline than that nf any other island in the i^niup, 

 a fact which has given to Oahu its valuable harbor facilities. 



Honolulu Harbor — Pearl Harbor. 



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

 stream, and about midway along the southern side of the island between ^laka- 

 puu and Barber's Point, we find the mo.st important harbor in the group. It is 

 foi-mcd b\- a sight indentation of the coast-line and is i)r(itecte(l by a coi'al reel' 



