jgg CALCAREOUS MANURES— APPENDIX. 



much more recently formed than the miocene. With neither of these is it 

 necessary to encumber this article. 



The different periods of time of these two different deposites 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 iew kinds 

 belonging to 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 also to the miocene marls. According to the highest geo- 

 logical authority, the race of animals whose remains formed the latest of 

 these deposites, were mostly extinct before the creation of man. 



Although it might be more conformable to regular or scientific arrange- 

 ment to commence a general description with the older and lower deposite, 

 the eocene marls, yet it will better suit the purpose of agricultural instruc- 

 tion to reverse the order, by describing first the miocene marls, as the 

 highest in the series and the first reached, and by very far the most abun- 

 dant and extensively accessible, and which therefore, though usually less 

 powerful for fertilization, are much the most important to agriculture in 

 Virginia in general. I shall therefore proceed first to treat of the miocene 

 marls, which are the only kinds known in Virginia with the exceptions of 

 the two comparatively small districts of eocene marl which will be hereaf- 

 ter treated of in their order. 



MIOCENE MARLS. 



When my investigations and practical labors on this subject were com- 

 menced, more than 24 years ago, the existence of marl of any kind had 

 been noticed in lower Virginia at but a few places, where naturally exposed 

 along steep river banks, and where cut through by deep ravines, and thus 

 rendered conspicuous ; and the deposite was supposed to be very limited, 

 by the few persons who had ever cast a thought upon the subject. But 

 the attention and observation subsequently directed to the search, soon 

 showed that the quantity was very far more extensive ; and now, though 

 not generally near the surface of the earth, nor every where accessible, it 

 seems probable that beds of fossil shells under-lie much the greater part of 

 all the region between the falls of the rivers and the sea-shore. Except 

 at or near the places where exposed on the surface, as above mentioned, 

 the overlying earth is generally 20 or 30 and even sometimes 50 feet thick. 

 All the marl-beds appear to be nearly horizontal, and of course are the 

 most deeply covered under the highest lands, and are most easily accessi- 

 ble in deep depressions. The deposite dips gently towards the east, so that 

 it lies too deep to be visible near the sea-coast. At Norfolk, the marl has 

 been recently reached, in boring deep for water, at 40 feet below that low 

 surface. 



The marl is formed by the deposite and gradual accumulation of sea- 

 shells, mostly left where the animals died ; and the vacancies between the 

 shells were filled by the sand or clay, or mixtures of both, with fragments 

 of older shells, brought by currents and deposited in what was then the 

 sea. The remarkably perfect state of preservation of many very thin and 

 always fragile shells, and still more the many pairs of bivalve shells that 

 yet are found connected or in contact, prove that such shells could not have 

 been transported, or even much agitated, by the force of the water. But 

 other beds of marl, and also frequently the upper layers of such as have 

 been just referred to, show as clearly the action of currents, or of water in 

 violent and long continued motion, which served to grind down the shells 



