42 
POPULAE SCIENCE EEVIEW. 
make out. Tke mud at the moutk of tke Ganges, tlie Missis- 
sippi, tke Nile, and tke Rhine, may even be measured and 
weighed ; and it is certain that what is gained by the water is 
lost by the land, — at any rate, to a large extent, — but there our 
daily experience ends. We do not see what becomes of this mud; 
we do not know and cannot trace with our eyes the changes 
that must take place before it can again assume the form of the 
rock from which we know it to be derived. We can easily 
reduce rock to mud, but not easily convert mud into rock. This 
sort of knowledge we must obtain, if at all, by applying what 
we know concerning the general laws and methods of nature. 
We must study mud and detritus under various forms, and 
study also rocks and minerals under all the different conditions 
that are available. It is thus, and thus only, that we can hope 
to obtain a glimpse of the real history of strata. 
Now the first thing to be noticed is the total absence of any 
repose in the present state of things on the earth. Owing to 
the three forms of matter — solid, liquid, and gaseous, — earth, 
water, and air, — and the incessant change of temperature 
characteristic of the surface of the earth, there cannot possibly 
elapse one moment of time without motion in every direction ; 
and if motion, then friction, wearing and removal of surfaces 
in contact, and thus more heat and more motion in an endless 
circle. And what is going on at the contact of earth, air, and 
water, where we live and breathe, is not in any way confined to 
these limits. Earth, air, and water are in contact and in 
motion as far below, and out of our sight, as they are in the 
upper regions of the atmosphere, where the clouds are always 
chasing one another, and where rush the meteors so inces- 
santly presenting themselves, and so often crossing the earth^s 
path. It is not only from the rumbling of the earthquake, or 
its fierce and vehement eruptions of fire and ashes from the 
volcano, that we know this. Wherever the waters pour forth 
their life-giving streams, whether from natural or artificial 
sorn’ces; wherever the mine gives forth its metalliferous wealth; 
wherever, in a word, the intelligence of man can penetrate and 
lay bare the doings of nature, there we know there is constant 
motion and constant change. The electricity that we conduct 
along the wire to enable us to communicate instantaneously 
between distant points, is but an infinitesimal part of the cur- 
rent that circulates from pole to pole, beneath the surface of 
the soil. The effect that we produce so readily by a few dis- 
similar minerals in contact with some simple acid, cannot fail 
to be produced with ten thousand times greater result within 
the earth, where are all the materials at hand. What are the 
strata and the veins, and the vein contents, but a vast collec- 
tion of galvanic batteries, whose supply of metal and acid, and 
