348 DYNAMICAL GEOLOGY. 



water-action, if of sedimentary origin. The shallow-water origin of beds is 

 so generally true that thick formations in almost all cases are proof that a 

 slow subsidence, equal in amount to the thickness, was going on over the 

 area during the deposition ; and also that without such a subsidence the 

 making of thick strata or formations has rarely taken place. 



Such evidences of actual change of level are good, and have profound 

 significance. Geology has thus proved that : — 



1. Unequal changes have been in progress simultaneously in different 

 parts of the same continent. 



2. The changes in level have usually gone forward with extreme slow- 

 ness — by the few inches or feet a century, though sometimes also by 

 abrupt displacements. The former are termed secular changes, the latter 

 paroxys7nal. 



Another class of facts is represented by the following from Illinois : — 

 A section of the coal formation of Illinois, described by Worthen, con- 

 tains 16 coal-beds, large and small, separated by fragmental beds and lime- 

 stones containing abundant remains of marine life. The coal-beds indicate 

 eras of emerged land, the marine fossils, intervening eras of submergence, and 

 their, number shows that at least sixteen alternations between the two con- 

 ditions there took place in the Carboniferous period. Facts make it certain 

 that the great Interior Sea of the continent communicated at that time freely 

 with the ocean to the south. The same region thus went up and down, 

 changing the dry land outline and the sea depths ; and the changes went on 

 with extreme slowness, for coal-beds, as well as the much thicker marine 

 beds, were slow in accumulation. Facts of similar import are afforded by 

 all the successive formations, from the Cambrian upward, and alike on all 

 the continents. In explanation of such changes it may be questioned 

 whether subsidences over the sea-bottom may not have produced some or 

 all of these oscillations in level. As far as evidence can be obtained, the 

 changes were independent of movements in the ocean ; for the coal-beds of 

 Illinois and those of Ohio and Pennsylvania 'do not show that uniformity 

 of parallelism which this h3^pothesis requires. 



Further : changes of level are noio in progress, both of the slow secidar kind 

 and of the sudden or paroxysmal. The following sketch represents a case in 

 which a Roman temple has passed through a time of partial submergence 

 below the level of the Mediterranean. The temple is that of Jupiter Serapis 

 at Pozzuoli. It was originally 134 feet long by 115 wide ; and the roof was 

 supported by 46 columns, each 42 feet high, and five feet in diameter. Three 

 of the columns are now standing, and they bear evidence of submergence for 

 a considerable time to half their height. The lower twelve feet are smooth j 

 for nine feet above this, they are penetrated by lithodomous or stone-boring 

 shells, remains of which (a species now living in the Mediterranean) were 

 found in the holes. The columns, when submerged, were consequently buried 

 in the mud of the bottom for 12 feet, and were then surrounded by water nine 

 feet deep. The pavement of the temple is now under water. Five feet below 



