CHAP. xxn. INTERCALATION OF NEWLY DISCOVERED FORMATION. 449 



were formed when the bottom of the sea was subsiding. 

 This downward movement protects the new deposits from 

 denudation, and allows them to accumulate to a great thick 

 ness ; whereas sedimentary ma.tter, thrown down where the 

 sea-bottom is rising, must almost invariably be swept away 

 by the waves as fast as the land emerges. 



When we reflect, therefore, on the fractional state of the 

 annals which are handed down to us, and how little even these 

 have as yet been studied, we may wonder that so many geo 

 logists should attribute every break in the series of strata, 

 and every gap in the past history of the organic world, to 

 catastrophes and convulsions of the earth's crust, or to leaps 

 made by the creational force from species to species, or from 

 class to class. For it is clear that, even had the series of 

 monuments been perfect and continuous at first (an hypo 

 thesis quite opposed to the analogy of the working of causes 

 now in action), it could not fail to present itself to our eyes 

 in a broken and disconnected state. 



Those geologists who have watched the progress of dis 

 covery during the last half century, can best appreciate the 

 extent to which we may still hope by future exertion to fill 

 up some of the wider chasms which now interrupt the 

 regular sequence of fossiliferous rocks. The determination, 

 for example, of late years of the true place of the Hallstadt 

 and St. Cassian beds on the N. and S. flanks of the Austrian 

 Alps, has revealed to us, for the first time, the marine fauna 

 of a period (that of the Upper Trias) of which, until lately, 

 but little was known. In this case, the palaeontologist is called 

 upon suddenly to intercalate about 800 species of mollusca 

 and radiata, between the fauna of the Lower Lias and that of 

 the Middle Trias. The period in question was previously 

 believed, even by many a philosophical geologist, to have been 

 comparatively barren of organic types. In England, France, 

 and Northern Germany, the only known strata of Upper 



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