438 MIOCENE MARL. 



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-con- 

 tinued motion, which served to grind down the shells to small 

 fragments, and which also left, in shaping the surface of the marl, 

 the marks of whirlpools or other violent disturbance. From such 

 supposed causes might be expected such effects as many of the 

 various marl-beds actually exhibit. In different places, and some- 

 times in the same place, the shells and their fragments are found 

 of all sizes, and of all conditions of preservation ; and intermixed, 

 in various proportions, with such clay, or fine sand, as might be 

 suspended in or borne by currents, or waves of the sea ; so as to 

 form beds of every degree of texture and shade of colour. The 

 shells, and their fragments, or the carbonate of lime, are in 

 various proportions of quantity, from 10 per cent, (or even less in 

 rare cases) to 90 per cent, or more, of the mixture, or whole mass. 

 In different beds, and sometimes in contiguous layers of the same 

 bed, the shells are in every state of preservation or of decay ; from 

 that of being firm, and often entire in their calcareous structure, 

 and the most delicate parts of their beautiful forms preserved, to 

 that of being mostly broken down, and almost reduced to a coarse 

 powder, and sometimes even forming a homogeneous mass of still 

 finer particles, in which the forms of but few if any shells are dis- 

 tinguishable. The original bright and various colours of the shells 

 are lost, and they are nearly all white a few of the hardest kinds 

 only being brown or gray. The texture of the mass also varies, 

 from a loose sand to a firm body of almost stony hardness. The 

 earth intermixed with the shells is generally much more sandy 

 than clayey, and more especially in the poorer marls. Even when 

 the admixture of earth is clay, it rarely makes the marl appear the 

 least clayey in texture, or plastic or adhesive, because the clay is 

 usually but in small proportion to the shelly matter. Even when 

 the proportion of clay is great, the carbonate of lime, according to 

 its quantity and degree of reduction, counteracts the tendency of 

 the clay, and prevents the mass being tough, adhesive, or obdurate. 

 The colour of the miocene marls is also various generally either 

 pale yellow or dingy white, or blue, sometimes bright, but more 

 often a dull blue, or ash colour. The richest marls, of homogene- 

 ous texture, are nearly white when dry, and approach in appear- 

 ance to a coarse or impure chalk. 



The shell marls of Virginia are confined almost entirely to the 

 tide-water region, or the space eastward of the granite which forms 

 the falls of all our eastern rivers. But near Petersburg (on the 



