Dana and Chambers on Ancient Sea- Margins. 219 



thrown up. The muddy deposits often afford more or less 

 of animal life of various kinds, and among them shells are 

 common, worn or unworn. 



The muddy deposits containing molluscs, and those beach- 

 accumulations that include worn shells, will, together, con- 

 stitute a cei*tain proportion of a line of coast, and the former 

 a larger proportion of the estuaries. The proportion for any 

 coast should be ascertained, that it may be used as a key to- 

 wards studying the accumulations over the country back. 



5. There is a minimum width for long arms of the sea, 

 which should be compared with the width of interval between 

 opposite terraces or rivers. 



Allowing now for a large part to be carried off by waters 

 after an elevation, it is evident that there will still remain, 

 in some parts among sea-coast formations, indications of 

 their marine origin. Such evidence is yet to be found in our 

 upper terraces ; and until it is obtained, there is no safety in 

 the inference, that the terraces, extending as they do through- 

 out the interior of a whole country, are marine. It will be 

 time enough to speculate on the facts when they are ascer- 

 tained. The terraces consist of material of all kinds, from 

 the coarsest pebbles to the finest silt ; and, assuredly, the 

 sea, if the source, must have left some traces of itself and its 

 productions, which may be detected where such extensive 

 deposits remain undisturbed. 



The writer has observed terraces along the rivers of Ore-. 

 gon and California, as well as in the eastern portion of our 

 country, and for some years has been hoping to take up the 

 subject for special study, feeling assured that a phenomenon 

 which so pervades a continent, — as is evident throughout the 

 United States, from east to west, must afford conclusions of 

 the highest interest to geology. Mr Chambers' work contains 

 much valuable material, shewing at least the wide distribu- 

 tion of terraces over his own country, as well as to some ex- 

 tent in other lands. But more study is required before the 

 great body of the facts he has collected are made available to 

 science. The subject should be entered upon in the manner 

 exemplified by that accurate and laborious Scotcli geologist. 



