CALCAREOUS MANURES— APPENDIX ^09 



tance to it, that he has referfed to it in several of liis subsequent publications. 



The most ready and certain mode of distinguishing eocene marl, is by 

 reference to some of the shells belonging to this kind, and which are never 

 found in niiocene marls. There are many such ; but the most common 

 and well marked are the two following: 1st, the curdila planicosla, a 

 bivalve white shell, having numerous regularly formed ridges running from 

 the point at the hinge of the valves to the circumference of the outer or 

 opening parts, and widening as the ridges extend — both valves alike, and 

 having outlines approaching to circular — sometimes seen more than three 

 inches across and the connected valves nearly two inches through, but 

 generally of much smaller and various sizes. 2d. The ostrea sellcpformis, 

 or saddle oyster, a curiously and variously contorted brown and very 

 hard bivalve shell, the larger valve of which approaches the shape and 

 reversed curves of a saddle. This shell is sometimes found more than 

 five inches in length. Both of these shells are abundant, especially the 

 cardita -planicosla, in this particular bed of eocene marl, and also in the 

 upper part of all the other eocene marls since known elsewhere in Virgi- 

 nia. Without reference to these, or to some other characteristic shells, 

 the eocene marl might not always be distinguishable by its texture or 

 general appearance from the miocene. And even these two shells, the 

 most abundant and characteristic of the eocene formation generally, are 

 neither to be found in the lov/er layers. 



For some years after the first discovery and application of this calcareous 

 eocene marl on Coggins Point farm, it was not known to exist elsewhere. 

 For even where then visible, and at later times used, its different character 

 was neither known nor suspected by its proprietors. As chance furnished 

 to me opportunities of seeing the beds, or as small specimens of the marl 

 were sent to me for examination, I gradually came to know the greater ex- 

 tent of this bed. It is now known at various points in an area of about 

 tweh^e miles in length, from east to west, and eight or ten miles wide, which 

 area takes in parts of the counties of Prince George, (which has much the 

 larger portion,) Charles City, and the lower point of Chesterfield. And in 

 this area also is the broad bed of James river, and the lower parts of its 

 considerable tributaries, Appomattox river, and Bailey's, Powell's, and Her- 

 ring creeks. The marl is exposed to view on the southern side of James river, 

 at the following several points: Coggins Point, Maycox, (a mile below, and 

 the most eastern exposure known,) Tarbay, Wm. H. Harrison's farm, and 

 Beaver Castle, all above on the river— Eelbank and Hawksnest, (the most 

 southern exposure,) on Powell's creek— the Old Court-house tract and Spring 

 Garden farm, both on Bailey's creek, and the latter from one to two miles 

 above the head of its tide, and three miles south of the Appomattox where 

 opposite. The last is the most western exposure. On the northern side 

 of the Appomattox, it is seen in the river bank at Bermuda Hundred, and 

 north of James river, and of Herring creek, at Neston and Evelynton. 



Through nearly all this large area, this bed of marl preserves remark- 

 able uniformity of appearance, texture, chemical character and composi- 

 tion, and even of the thickness of the stratum, and of the succession and 

 variations of character of the several smaller layers of the general body. 

 The bed lies nearly horizontal, but dips slightly and irregularly eastward 

 and northward. At Coggins Point, its lower part is 10 to 12 feet above 

 high tide, while at Maycox, a mile to the east, and at Evelynton three miles 

 north, it is lower than high tide mark. Yet not so much difference of ele- 

 vation as this is seen in all the greater extension westward to Bermuda 

 Hundred. The stratum varies from 4 feet to 10 feet thick, being thinnest 



