436 SHELL MARL OF VIRGINIA. 



of layers of drifted and barren sands or clays, or both, with a sur- 

 face-soil usually poor and thin. Farther south, the denuding and 

 destructive power of this flood was so much less, that the shell 

 bed is left several hundred feet in thickness. In Virginia, the re- 

 maining bed is in most cases less than fifteen feet in thickness, 

 and rarely much more. 



As there is good reason for believing that all the present great 

 tide-water region of the countries last named was formerly the 

 bottom of the ocean, for an immense length of time, we may in- 

 fer that the whole was originally covered, to greater or less depth, 

 with a continuous bed of shells. Wherever this formation is now 

 wanting, it must have been removed by the subsequent washing 

 flood, previous to its later action of depositing the enormous bed 

 of drifted earth, which overlies the shells, or their former place. 



The fossil shell beds of Virginia, which will be the main subject 

 to be treated of here, may be again conveniently divided, for de- 

 scription and observation, into two kinds, of (B 1) Miocene, and 

 (B 2) Eocene. These terms (with others) were introduced by 

 Professor Lyell, and designate the formations of different geologi- 

 cal eras. As they are now of general acceptation by geologists, 

 and also are generally understood by agricultural readers, these 

 terms will be convenient, and will be here used to designate the 

 different marls to which they belong. If the difference between 

 these two kinds were merely geological, or in regard to comparative 

 ages of formation, or to the respective fossils of each, it would be 

 useless to preserve it in writing on agriculture, however marked the 

 difference, and however interesting to the geologist. But there is 

 also a difference of agricultural character and value in these two 

 kinds of marl. In relation merely to each other, the terms eocene 

 and miocene may be sufficiently understood as the older and newer 

 formations. But it will not do as well to substitute the latter 

 terms, because, though correct as to each other, they are not so 

 generally, or in relation to other marls and geological formations. 

 For there are some (of secondary formation) much older than the 

 eocene, and others (older and newer pliocenes and post-pliocene) 

 much more recently formed than the miocene. With neither of 

 these is it necessary to encumber this report, by other than slight 

 notice, as neither are known in Virginia; nor elsewhere do they 

 present important differences of agricultural character and qualities. 



The different periods of time of these two different deposits of 

 shells were very remote from each other, and the latest of them 

 was also very remote from the present time. In the miocene marl 

 of Virginia, or later of the two, of the numerous species of shells 

 found, there are but few kinds belonging to races of animals known 

 or believed to be yet existing ; and in the eocene marl of Virginia 

 there are almost none that now exist, and very few that belong 



