GEOLOGICAL INTER-REGNA, . 2I 
able amount of mud, in the course of 100,000 years about 
600 feet would be deposited. At the bottom of the open 
sea, far away from the coasts, during this long period only 
some few feet of mud would be deposited. Even on the 
sea-shores where a comparatively large quantity of mud is 
deposited the thickness of the strata formed during the 
course of a century may after all amount to no more than 
a few inches or lines when condensed into solid stone. In 
any case, however, all calculations based upon these com- 
parisons are very unsafe, and we cannot even approximately 
conceive the enormous length of the periods which were 
requisite for the formation of the systems of neptunic 
strata. Here we can apply only relative, not absolute, 
measurements of time. 
Moreover, we should entirely err were we to consider the 
size of these systems of strata alone as the measure of the 
actual space of time which has elapsed during the earth’s 
history. For the elevations and depressions of the earth’s 
crust have perpetually alternated with one another, and the 
mineralogical and paleontological difference—which is per- 
ceived between each two succeeding systems of strata, and 
between each two of their formations at any particular spot— 
corresponds in all probability with a considerable intermedi- 
ate space of many thousands of years, during which that 
particular part of the earth’s crust was raised above the 
water. It was only after the lapse of this intermediate 
period, when a new depression again laid the part in ques- 
tion under water, that there occurred a new deposit of 
earth. As, in the mean time, the inorganic and organic con- 
ditions on this part had undergone a considerable transform- 
ation, the newly-formed layer of mud was necessarily com- 
