ON THE PROGRESS) OF GEOLOGY. 399 
the centre, and matter in a high state of fusion filling the space 
between both, have been advocated by other scientific men, and 
mathematical proofs in support have not been wanting. How- 
ever, objections apparently fatal to them all have been brought 
forward at one time or another by physicists, astronomers, or 
geologists, according to their particular line of study, and we 
can therefore only wait patiently and follow attentively the care- 
ful researches continued in all civilized countries, applying at 
the same time every new discovery to the elucidation of a prob- 
lem, the more tantalizing as its solution has for many years 
appeared to be within our grasp. 
The great hopes that the deep borings lately obtained 
in artesian wells, or careful temperature observations in 
deep mines, would supply us with some material for ad- 
vancing this question, by offering important and reliable 
data of a uniform character, have not been fulfilled. It appears, 
on the contrary, from the deep borings at Sperenberg, in Ger- 
many, reaching nearly to 4200 feet, that the increase of heat 
exhibits a remarkable retardation of its rate the deeper we 
descend. And even if we take convection and conductivity of 
the rocks into account, there are scarcely two localities where 
the same ratio of increase in the temperature has been observed, 
in some that ratio being more than treble that of others. There 
may once have been a uniform cooling of the original crust of 
the earth; now almost entirely removed or remodelled, but there 
is no doubt that this difference in the increase of temperature 
depends now either upon local generation of heat by hydro- 
chemical action or mechanical agencies of enormous power still 
at work. Thus in localising the variable increase of tempera- 
ture, the vera causa both for the crumpling and metamorphism 
of rocks, for the formation of mountain chains, as well as for the 
origin of volcanic action, might be traced with more reliance, 
than to seek to establish a general law that most probably no 
longer existed when the strata accessible to our examination 
were formed. 
Leaving the dominion of theory and returning to the actual 
work of the geologist in the field, I need scarcely say that the 
task already accomplished has been truly gigantic. Patient 
research in the civilized countries of Europe, in the United States 
of North America, and most of the: English colonies, as well as 
the work of travellers to almost every part of the globe—of the 
latter I wish only to allude to Baron von Richthofen’s excellent 
late researches in China—have made us acquainted with such 
remarkable and innumerable data, that it is impossible for any 
man, however studious he may be, to gain more than an imper- 
fect knowledge of the material already accumulated. 
The relations of the plutonic, metamorphic, sedimentary, and 
volcanic rocks to each other have been clearly defined, and most 
valuable facts have been brought together, from which the past 
history of our globe is being constructed, while the palzeontolo- 
gist has done his work equally well in classifying the wonder- 
