242 J. D. Dana on Volcanic action at Mauna Loa. 
by the very nature of volcanic forces. The fractures of sucha 
vast mountain as that of M. Loa—14,000 feet high, and 50 miles 
in diameter at base, must have their starting point from a depth 
as far down at least as the water level; aud if such a fracture 
opens so as to make a fissure of a mile anywhere at the surface, 
it will have many times that length below ; moreover, owing to 
the strain producing such a rupture within, there would probably 
be several other lines of surface fracture, along the slopes be- 
tween the first outbreak and the sea. 
The lateral cones formed along the line of such an eruption 
mark the points of widest fracture or the intersections of frac- 
tures, and are not the sole sources of the lava. 
. A volcanic mountain whose average slope is 6° to T° may 
have eruptions extending from the summit to the base.—The 
three great eruptions of M. Loa directly sustain this fact. More- 
over, as Mr. Coan states, while the average slope is 6° to 7°, 
there are many intervals where the angle is 25° to 30°, and some 
of 49° and 60° or more, down which the lavas poured, and where 
they afterwards hardened. We have not however definite state- 
ments as to the thickness of the lava stream along these steeper 
declivities. ‘The facts at least set aside the notion that the lavas 
of a crater are thrown out at an angle not exceeding 3°, and that 
a higher angle is a result of elevating forces below at centre, 
thumping it upward. Elevation by uplifting action beneath 1s 
part of the history of every lava cone. But it begins, as the 
facts in Kilauea prove, with the first formation of the cone, and 
continues with its progress—growth by overflow and elevation 
going on together; and if there be any difference, the uplifting 
action should diminish (instead of decrease,) as the crater loses 
its original activity. 
4. The basaltic character of the lavas.—The rock, Mr. Coan 
states, is similar to that of the other eruptions. Indeed the re 
cent lavas all over Hawaii are much alike; they vary 10 the 
amount of chrysolite, but otherwise are quite similar. here 
cin- 
ders, which mark so strikingly the spiteful Vesuvius oe almost 
oa. 
In- 
