AND FRESH-WATER FORMATIONS. 317 



occur a change of the organic fossils in the collection 

 of strata formed. 



And in each of such cases, hypothetically put at 

 present, there must occur purely terrestrial alluvia. 

 The uppermost portion of a drained or filled lake 

 must always be of this nature. But this is not all. 

 These alluvia are progressive, and often at more 

 points than one. And under this progression, they 

 are subject to be overwhelmed by inundations, be- 

 coming thus buried under the finer materials which 

 form the bottom of such lakes, of whatever nature. 

 This fact occurs daily : and thus are terrestrial alluvia 

 often deeply covered by materials which are properly 

 aquatic, inasmuch as they form the subaquatic de- 

 posits. This is common with peat, often deeply 

 covered by sand or mud, and thus even producing 

 the alternations familiar in mountainous countries. 

 In such alluvia also, there may occur the remains of 

 terrestrial animals, whether from inundations or other 

 causes ; and hence may such remains alternate with 

 mud or sand, or with those substances after conver- 

 sion into marl, shale, limestone, or sandstone. 



Now what I have thus stated hypothetically, con- 

 sists of a series of facts, which, in some place or 

 other, all occur. They are the facts on which a 

 theory is always safely founded : it would be well if 

 there were many as well supported. And they ex- 

 plain every essential circumstance which has ever yet 

 been observed in such tertiary deposits as have de- 

 manded no other or accessary causes for their present 

 condition. Except the occasional presence of gyp- 

 sum, there is nothing unexplained, thus far. There 

 does not even remain one of those imagined mysteries, 

 respecting the mixtures or alternations of marine and 

 fresh water fossils, or the occurrence of the skeletons 



