GEOLOGICAL INTER-EEGNA. 21 



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 tlie 

 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 palseontological 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, thaj; 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- 



