OF THE HAWAIIAN ISLANDS. 441 



that it ' leaned the other way,' — thus flowing down a bank until it came to where the rock 

 retreated, it would follow the inward curve in a thin layer like molasses, adhering to the rock 

 and thus cooling." ' 



I have myself seen precisely such cases as Mr. Coan describes, and the consolidated lava 

 often looks like mud slowly oozing through the cracks and down the surface of a perpendic- 

 ular wall. I have seen a bed of lava several feet thick and quite compact, which had 

 solidified at an angle of 35°. Cases have been cited where the lava has flowed over a bank 

 in a thin stream which has formed layer after layer until the angle has been reduced to 3°. 



Cooling of Lava. 



It has been shown that the lava cools very rapidly on the surface, so that a stream may 

 be walked upon a few hours after it has been incandescent. The lava on the surface of the 

 Halemaumau crusts over almost instantly after the action ceases, and the fragments of lava 

 thrown into the air cool almost before their rapid motion can change their form. Instances 

 have been cited where the lava, dashed upon a tree by a passing flow, has consolidated in 

 rings around the branches without burning into the bark. The casts of the trees, occurring 

 quite through a bed twenty feet thick, show that where the caloric can escape through any 

 good conductor the lava becomes solid. The crust is however a most excellent non-conduc- 

 tor, and under its protection the lava may run for months, or remain heated for years. The 

 flow of 1810 was steaming for more than ten years after it ceased to run, and while the 

 other recent flows have cooled in a much shorter time, it is owing to their being quite thin 

 and cracked. 



The more rapidly lava cools the more vitreous will be its texture, as is shown in the 

 Pele's hair, in the drops of lava thrown from the lake into the air, which are as vitreous as 

 obsidian, and in the surface of lava-streams. 

 I obtained many specimens of crust from the 

 wrinkled streams, and while the inside was quite 

 porous, the outer surface, to the depth often of 

 half an inch, was quite compact and vitreous, as 

 will be seen from the impression of a section 

 of crust. The lava poured out of the pools in 

 Kilauea, and some of that in the summit erup- 

 tions, cools very rapidly, and the whole surface 

 much resembles the thin glass flakes produced 



, „ , ,., , Fig. 47. Impression of lava crust. 



in the process of stamping bottles at a glass- 

 house. Where melted lava is thrown into water, it is broken and granulated into a glossy 

 coarse sand or gravel. 



The temperature of the Hawaiian lavas has never been determined, although from their 

 extreme liquidity it is probably very high. 



Formation of Pit Craters. 



Kilauea, and the similar craters on Maunas L6a, Hualalai, and other mountains of the 

 group, are simply orifices left by the overflowing lava, and by no means the remains of 



1 Silliman's Journal of Science, [n. s.] vol. xxi. p. 142. 



BOST 



. SOC. NAT. HIST. Vol. I. Pt. 3. 112 



