MIOCENE MARL. 437 



also to the miocene marls. According to the highest geological 

 authority, most of the races of animals whose remains formed the 

 latest as well as the earliest of these deposits, were extinct before 

 the creation of man. 



Although it might be more conformable to regular or scientific 

 arrangement to commence a general description with the older and 

 lower deposit, the eocene marls, yet it will better suit the purpose 

 of agricultural instruction 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 abundant 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 to me in Virginia, with the exceptions 

 of the two comparatively small districts of eocene marl ; which will 

 be hereafter treated of in their order. 



Miocene Marls. 



"When my investigations and practical labours on this subject 

 were commenced, more than twenty-four years ago (in 1818), the 

 existence of marl of any kind, or rather its shells, obvious to the 

 sight, 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 deposit 

 was supposed to be very limited, by the few persons who had ever 

 cast a thought upon the subject. But the attention and observa- 

 tion subsequently directed to the search, soon showed that the 

 quantity was very far more extensive ; and now, though not gene- 

 rally near the surface of the earth, nor everywhere accessible, it 

 seems probable that beds of fossil shells underlie 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 (drift) earth is generally 20 or 

 30 and sometimes even 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 accessible in low de- 

 .pressions. The deposit 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 be- 

 low that low surface, and, of course, much below the sea. 



The marl is formed by the deposit 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 tide and currents, 

 and deposited in what was then the bottom of the sea. The re- 

 markably perfect state of preservation of many very thin and 

 37* 



