1336.] 



FARMERS' REGIST E II . 



629 



black color. At the base of the cliffs on the 

 James and York rivers, thisstratum may be traced 

 continuously for considerable distances, rarely 

 rising more than two or three feet above the level of 

 the water, and presenting an even horizontal out- 

 line. In the deep ravines, and low down in 

 the banks of shells, generally, throughout this re- 

 gion, a similar dark bluish green argillaceous sand 

 is observed, enclosing frequently a great number 

 and variety of shells. This constitutes what is 

 usually denominated blue marl, which from the 

 sol't condition of the shelly matter which it con- 

 tains, as well as the predominance of clay in its 

 composition, is found peculiarly beneficial when 

 applied to the more arenaceous varieties of the 

 soil. Many highly valuable marls extensively in 

 use are oi* this description. 



The very general existence of the lower stra- 

 tum, above described, forms an interesting and 

 prominent feature in the geology of the Miocene 

 Tertiary districts, as well of eastern Virginia as of 

 Maryland. Throughout all the upper fossil iferous 

 strata, as well as in the argillaceous beds just 

 mentioned, will be found disseminated, greenish 

 black grains of silicate of iron and potash, identi- 

 cal with those already described as existing in the 

 stratum immediately overlying the shells, and ha- 

 ving the same form and composition with the 

 granules contained very abundantly in an older 

 formation, both in this country and in Europe. In 

 some beds of the marl or shells, these particles so 

 much abound as to give a very decided color to 

 the whole mass. In specimens from James City 

 and York counties, as much as thirty- five per 

 cent, of the green sand has been found, and occa- 

 sionally shells are seen filled with this substance 

 almost alone. 



The surface of the strata containing shells is 

 usually irregular. Sometimes it rises abruptly, in 

 the form of a hillock, then it is scooped out into de- 

 pressions of a few feet in depth. These irregu- 

 larities, however, are apparently of two kinds; the 

 one the original form of the deposite, the other 

 produced by denudino; action upon the surface. 

 Thus in many places the same stratum may be 

 remarked, rising with more or less abruptness, 

 then again descending, and perhaps preserving a 

 pearly horizontal line for some distance, marked 

 at its upper surface by a clear and unbroken out- 

 line, and presenting no indication of violent abra- 

 sion from above. In other places, and this is 

 a very frequent occurrence, deep and irregular fur- 

 rows and cavities are seen, such as would natu- 

 rally arise from the action of the currents and ed- 

 dies of a large mass of water in rapid movement. 

 Whether from this cause, or from the Gradual dis- 

 solving action of percolating water, sinks exist in 

 this region in many places, though they are by no 

 means as numerous or extensive as in the lime- 

 stone districts. 



Having thus given an account of the nature 

 and arrangement of the strata overlying the shells, 

 as well as those in which they are embedded, we 

 will now describe the general condition and dispo- 

 sition in which the shells occur. 



Condition of the shells in the Tertiary Deposiles. 



In general the state of the shells, and their ar- 

 rangement in the earth, are such as to indicate 

 their tranquil deposition at the spots in which they 



are found. Thus the corresponding valves, are 

 very often found together and closely shut. Many 

 of the smaller shells, such as Area cenetnaria, Ar- 

 ea incile, Nuculas, Venericardia alticosta, and 

 Chama congregata, which are most usually found 

 thus, are olten either entirely empty, or contain a 

 small quantity of clay that is quite impalpable, 

 indicating plainly that they have been exposed to 

 no violence, and that only such solid matter as 

 could pass between the edges of the closed valves 

 had obtained access to the interior. Whenever 

 such shells, however, have been previously drilled, 

 as is very frequently the case, even with the largest 

 and thickest shells,the interior is found entirely filled 

 with sand, clay, green sand, and small fragments 

 of shell. In most cases the larger species of 

 shells, even when their valves appear to be in ac- 

 curate juxtaposition, is thus filled, and in this case 

 it cannot be supposed that the contained matter 

 has entered through the holes thus drilled, since in 

 many instances shells of considerable magnitude 

 are found imprisoned within. Such shells, no doubt, 

 afler the death of the animal, remained open, or at 

 least partially so, and received the sand, clay and 

 other materials which they contain, by the gentle 

 action of the waves. The ligament at the hinge in 

 the mean time would decay, until at length, yield- 

 ing to the pressure of the accumulating maiter 

 above the shell, in favorable circumstances, would 

 collapse into its natural closed condition. 



The very common occurrence of the valves in 

 juxtaposition, is a striking proof, that during' or 

 subsequent to their deposition, they have not been 

 exposed to violent agencies. This becomes even 

 more remarkable in the case of such shells as the 

 Panopea reflexa, which almost in every instance 

 is found with the valves properly united. The 

 connection between the two valves in this shell is 

 the slightest imaginable, after the destruction of 

 the natural organic bond, and an inconsiderable 

 force would have sufficed to separate and break 

 the valves. 



The admirable preservation of the shells in 

 many cases is also an interesting fact, and affords 

 another evidence of the absence of all violent 

 agencies at this period. The most fragile species 

 of Natica, delicate Telliiue, Mactra Tellinoides, 

 the shell and processes of the Crepidula, the mi- 

 nute and sharp angles of the Fusus Tetricus, the 

 thin and hollow Fissurella are found in perfect 

 preservation in many places. The state of the 

 shells seems to depend chiefly upon the mechani- 

 cal texture and chemical character of the mate- 

 rials with which they are mixed, and of which the 

 overlying stratum is composed. In the moist blue 

 clay, the shells are irenerally found in a very sofi 

 condition. In a highly ferruginous clayey bed 

 they are found either partially or entirely dissolved, 

 and beautiful casts remain in their stead. 



In many places entire banks occur, composed of 

 casts of Chama and other shells, sometimes sepa- 

 rate, sometimes cemented together so as to form 

 a species of rock. These appearances occur 

 chiefly near the surlace, and when the soil is po- 

 rous and ferruginous. The casts thus formed, 

 often consist chiefly of carbonate of lime, and in 

 many specimens as much as eighty per cent, of 

 this substance is found. Casts of this kind belong 

 mostly to the smaller shells, and by far the most 

 common are of the Chama congregata. These, 

 as already stated, are often found nearly or quite 



