APRIL 4, 1884.] 
sage through the forest, the flow encountered 
trees of all sizes, to a yard or more in diameter. 
Flowing around these, it solidified sufficiently 
to retain a complete mould of the trunks before 
they burned off. By means of these upright 
moulds or wells it is comparatively easy to 
measure the depth of the lava at any point 
throughout this portion of its length. This I 
found to average about twenty feet, though 
varying very much in particular instances, ac- 
cording to the nature of the surface over which 
it flowed. 
As their trunks burned off, the trees fell 
upon the surface of the still plastic lava with 
sufficient force to impress upon it a mould of 
a portion of their outline. 
In both the vertical and horizontal moulds 
a peculiar impression was made upon the sur- 
face of the lava in contact with the tree. It 
took on a honeycomb structure, presenting a 
series of indented squares. ‘The cause of this 
peculiar form I have been unable to determine. 
The indentations are certainly not the impres- 
sion of the bark of the trees, and they are 
altogether too regular to be the result of the 
expansion of gases. 
The general structure of the flow, through- 
out its entire length, is that of a long, central 
tunnel with numerous lateral branches. Flow- 
ing lava cools very rapidly, — indeed, so quick- 
ly, that I have often passed over the surface of 
that which I had seen flowing fifteen minutes 
before. Being a good non-conductor, the heat 
of the inner portions is long retained after the 
surface is once solidified. In this way a long 
central tube is formed, from the lower end of 
which the lava continually flows, while it con- 
tinually extends the length of the tube. The 
pressure along the tube is constantly becom- 
ing too great for its sides to bear, and lateral 
off-shoots are formed, increasing the width of 
the flow. These lateral tunnels usually fill up, 
and finally become solid. The central tunnel, 
however, remains hollow throughout a large 
portion of its length, and may often be traversed 
for long distances after the flow has become 
cool. On my first visit to the flow, the top 
of this central tunnel had fallen through in 
many places, and I was able to look into it for 
some distance; but in every case I found the 
heat still too intense to allow me to descend 
into it. At later visits this became possible. 
The roof is commonly rough, a broken surface 
of lava, but in many cases is smooth and shiny, 
and covered with numerous stalactitic forms, 
seldom more than an eighth of an inch in diam- 
eter, but often having a length of five and six 
inches. 
SCIENCE. 
415 
The stalagmite form was very rare, and only 
in one case did I find any of large size. In 
this instance the lava had flowed over a small 
precipice in a sheet in such a manner as to 
leave an opening between the sheet and the 
face of the precipice. Directly at the foot of 
the precjpice were two peculiar stalagmitic 
forms made of drippings of lava about a 
quarter of an inch thick and three and four 
inches long. The larger of these was about 
a foot in height; the smaller, not more than 
half that size. These had evidently been 
formed by the lava covering a small spring, 
the steam generated from which had kept the 
lava above in a semi-plastic condition for some 
time. These specimens attracted much atten- 
tion, as nothing of the kind had before been 
found. They may now be seen in the museum 
of the Boston society of natural history. 
Owing to this peculiar property of lava to 
form tubes for itself in which to flow, it has 
the power to flow over small elevations, thus 
presenting the phenomenon of a liquid flowing 
up hill. It has the power to continue this till 
the pressure becomes too great for the strength 
of the sides of the tube. 
The opposite view is from a photograph taken 
on the spot during the flowing of the lava. It 
shows the lava in the act of flowing over a pre- 
cipice about fifteen feet in height. Each of the 
small streams seen trickling down the face of 
the rock is red-hot lava. This illustrates the fact 
that lava flows at the steepest angles, which is 
sometimes questioned. In this view the lava 
is flowing into a small pool of water, the re- 
sultant steam from which is seen arising like 
the mist of a cataract. This depression was 
afterward so entirely covered by the lava, that 
at my visit the spot could not be distinguished. 
It would be interesting, if possible, to give 
an approximate estimate of the amount of lava 
which was forced above the surface during this 
eruption ; but with the present data it is impos- 
sible. No survey of the flow has been made, 
and it is exceedingly difficult to estimate its 
dimensions by the eye. As a very rough esti- 
mate, I should place it at something more than 
five hundred million cubic yards. 
| Gro. H. Barron. 
THE STATE OF EXPLORATION IN 
AFRICA. 
THE exploration of the dark continent continues 
with unabated vigor, in spite of the occurrences in the 
Sudan which nevertheless, in the form of disturbing 
rumors, must, even at a considerable distance, exer- 
cise an evil influence upon the native population. 
