112 J. D. Dana — Cause of the form of the Mt. Loa Dome. 



Sagging from pressure and consequent crushing has been 

 made a cause of a single concavity between the top and the base 

 of a volcanic mountain, and mathematical calculation has found 

 a conformity between physical law and the shapes of such 

 mountains in Japan and America. But there can be no crush- 

 ing from gravitational pressure in a mountain made almost 

 solely of lava ; and it is hardly a possible result in any existing 

 cone if made up even one-half of lava-streams, braced as they 

 are by dikes. 



. The following are the mean slopes of Mt. Loa from the sum- 

 mit along different radii. The distances made the basis of the 

 calculations are taken from the Government map : 



S.S.W. to the southern cape 1 : 13-1=4° 22' 



S.E. by S. to the indented Kapapala shore 1 : 9=6° 20' 



S.E. to foot of slope W. of Kilauea 1 : 9-12 = 6° 15' 



E.N.E. to shore at Hilo 1 : 14-86=3° 51' 



W. by S. to western shore 1 : 8-11 = 6° 43' 



N. by E. to plain between Loa and Kea 1 : 9 to 1 : 10 = 5° 50' to 6° 



In a circle of five miles around the summit crater the mean 

 slope is about 3° : the mean depression to the eastward at the 

 perimeter of the circle is about 1400 feet. 



From Kilauea to the eastern cape, 28 miles, the slope is 1 : 36^ 

 = 1° 35'. 



The fact that Mt Loa as well as Kilauea were made over a 

 great fissure has given an oblong and approximately elliptical 

 or ovoidal form to all the upper contour lines of Mt. Loa. 

 Further, the bend in the longer axis of the summit crater, mak- 

 ing the concavity to the eastward, is also expressed, according 

 to the large government map, in the form of the upper part of 

 the dome. 



At what period in its history, Mt. Loa left off superfiuent dis- 

 charges and took to having only the effluent, or those through 

 fissures, it is impossible to say. But as the walls both of Ki- 

 lauea and the summit crater are made up of the edges of lava- 

 streams to the very top, it would appear that summit overflows 

 from the crater may have continued in each to a comparatively 

 recent time. It is remarkable that the north and west walls of 

 Kilauea, which show well the stratification from top to bot- 

 tom, have almost no intersecting dikes. 



In the following paper, the relations of the Kilauea volcano 

 to Mt. Loa will be considered, and the question as to the effects 

 of volcanoes on the depths of the ocean. 



[To be continued.] 



