D. Forbes — On Volcanos, 321 



reported to have made the sky as dark as night in the Barbadoes ; 

 and in Iceland in 1766 the air became so charged with ashes for a 

 distance of 150 miles around Hecla, that even the brightest light 

 could not be distinguished at a few yards. 



Amongst the still active volcanos, we meet with some whose 

 craters are several miles in diameter, encircled by precipitous sides, 

 rising to even a thousand feet above the bottom of the crater when 

 at rest, which, as in the Sandwich Islands, may contain reservoirs or 

 rather lakes of liquid lava, two to four miles across, and at times 

 send forth rivers of molten stone several miles in breadth, extending 

 their fiery inundation to a distance of even 40 miles from the crater 

 whence they issued. In the eruption of Hualalia in 1801, a lava 

 current, after reaching the coast, poured out such volumes of lava as 

 to fill up a bay some twenty miles deep, and in its place extend a 

 headland some three or four miles further into the sea. 



The rate at which these rivers of molten stone flow is a very 

 varying one ; in 1805 the lava current from Vesuvius is said to have 

 run down the first three miles in four minutes,, yet only completed 

 its total distance of six miles in three hours ; and in 1840 that from 

 Mauna Loa advanced no less than 18 miles in two hours ; whilst, on 

 the other hand, it is recorded that during the eruption of Etna which 

 commenced in 1614 and continued many years, the lava stream only 

 completed a distance of six miles in 10 years, notwithstanding that 

 all this time it was seen to be in slow but almost imperceptible 

 motion. During the eruption of this volcano in 1865, I found, how- 

 ever, that at the edge of the current the rate of motion varied from 

 15 to 120 feet per hour, according to local circumstances, whilst in 

 the centre of the stream the lava was evidently still more rapid in 

 its movements. The entire mass of a lava stream often advances 

 even when to the eye it would appear to have become quite solid. 

 Upon my throwing a heavy stone on to the top of a lava current so 

 far consolidated that the stone merely fixed itself into the surface 

 without sinking deeper, it was seen that the stone moved along with 

 the lava, which otherwise looked as if stationary. 



The surface of this lava consolidated and cooled with almost in- 

 credible rapidity. Notwithstanding the protestation of my guides, I 

 walked over lava currents when, at the same time, the fiery stream, 

 still flowing below, could be distinctly seen through the cracks in 

 the crust over which I passed. 



On this occasion, also, the stems of the pine-trees in the forest 

 which was destroyed by this eruption were converted into charcoal 

 as high as the lava reached, but the upper portions of the trees 

 then toppled over, and remained in an almost unaltered and un-r 

 charred condition on the top of the lava current, which had so 

 quickly cooled. The crust which forms on the top of the lava when 

 cooling, being an e;xcellent non-conductor, acts so efficiently in pre- 

 venting further escape of heat, that we find streams of lava requiring 

 many years and even ages to become quite cold. Dolomieu relates 

 that the lower part of the Ischia lava of 1301 was still hot in the 

 year 1785. 



