CLAY AND ITS MODE OF OCCURRENCE. 23 



form at a few points in the deposit, or only along certain layers, 

 but in other instances they have originated in all parts of the 

 mass both along the stratification planes, as well as in every joint 

 or crack. They thus permeate the clay deposit with such a net- 

 work of rusty, sandstone-like chunks, nodules, and strips as to 

 seriously interfere with the digging of the clay, and requiring 

 powerful machinery to break up the hard parts. A surface view 

 of a clay outcrop showing these iron crusts projecting above the 

 surface in the form of small ridges is shown in Plate II, Fig. 2 

 (p. 12). 



Concretions. — In some deposits the limonite or siderite (car- 

 bonate of iron) collects around nuclei, 1 such as pebbles or grains 

 of sand, and grows into more or less symmetrical ball-like con- 



:-r~-;Qr-°rb~--A~b-^ 



\\ " . • 



Fig. 17. 



Section showing occurrence of concretions in certain layers. 



cretions, which if large can be avoided or thrown out in mining. 

 These are most abundant in the weathered portions of the clay. 

 (Fig. 17.) They are not to be confused, however, with the 

 nodules and lumps of pyrite that are found throughout some clay 

 beds, and are of yellow color and glistening metallic lustre. These 

 latter, although of secondary origin, are not necessarily due to 

 weathering. 



In many calcareous clays concretions are specially abundant, 

 being found not uncommonly along lines of stratification (Plate 



T The way in which natural physical forces act, to bring about this segrega- 

 tion of chemical compounds of the same kind, is not yet satisfactorily 

 explained, although it is a common phenomenon. 



