CLAY AND ITS MODE OF OCCURRENCE. n 



The general character of sedimentary clays is more or less 

 influenced by the locality and conditions of deposition, which 

 enables us, therefore, to divide them into the following classes : x 



Marine clays. — This class includes those sedimentary clays 

 deposited on the ocean bottom, where the water is quiet. They 

 have, therefore, been laid down at some distance from the shore, 

 since nearer the land, where the water is shallower and disturbed, 

 only coarser materials can be deposited. Beds of clay of this 

 type may be of vast extent and great thickness, but will naturally 

 show much variation, horizontally at least, because the different 

 rivers flowing into the sea usually bring down different classes of 

 material. 



Thus, one stream may carry the wash from an area of iron- 

 stained clay, and another the drainage from an area of white or 

 light-colored clay. As the sediment spread out over the bottom, 

 the areas of deposition might overlap, and there would thus be 

 formed an intermediate zone made up of a mixture of the two 

 sediments. This would show itself later as a horizontal transition 

 from one kind of clay to another. These changes may occur 

 gradually or at other times within the distance of a few feet 

 (PI. II, Fig. i). 



The laminations produced by vertical changes are shown in 

 Plate XLVI, Fig. 2. 



The most persistent beds of this nature are found in the rocks 

 of the Silurian and Devonian systems, but beds of considerable 

 horizontal extent are found in New Jersey in the Clay Marl 

 series of the Upper Cretaceous. 



Bstuarine clays. — These form a second type of some import- 

 ance in New Jersey. They represent bodies of clay laid down in 

 shallow arms of the sea, and are consequently found in areas 

 that are comparatively long and narrow, with the deposits show- 

 ing a tendency towards basin shapes. If strong currents enter 

 the estuary from its upper end, the settling of the clay mud may 

 be prevented, except in areas of quiet water in recesses of the bay 

 shore. Or, if the estuary is supplied by one stream at its head, 



1 A person lacking geological training will not always be able to t£ll to 

 which one of the following classes a deposit belongs. 



