24 CLAYS AND CLAY INDUSTRY. 



IV). Many of the drift clays, though free from lime, show con- 

 cretionary lumps, and in some deposits they have been formed 

 by the deposition of lime carbonate around tree roots. In this 

 case they would be closely associated with weathering. 



Formation of shale. — Many sedimentary clays, specially those 

 of marine origin, after their formation are covered up by many 

 hundred of feet of other sediments, due to* continued deposition 

 on a sinking ocean bottom. It will be easily understood that the 

 weight of this great thickness of overlying sediment will tend 

 to> consolidate the clay by pressure, converting it into a firm rock- 

 like mass, termed shale. That the cohesion of the particles is due 

 mostly to pressure alone is evidenced by the fact that grinding the 

 shale and mixing it with water will develop as much plasticity as 

 is found in many surface clays. An additional hardening has, 

 however, taken place in many shales, due to the deposition of 

 mineral matter around the grains, as a result of which they become 

 more firmly bound together. 



In regions where mountain-making processes have been active 

 and folding of the rocks has taken place, heat and pressure have 

 been developed, and the effect of this has sometimes been to trans- 

 form or metamorphose the shale into slate or even mica-schist 

 (when the metamorphism is intense), both of which are devoid 

 of any plasticity when ground. The Hudson River slates found in 

 northwestern New Jersey owe their low plasticity partly to a slight 

 metamorphism, and partly to the deposition of cement around the 

 grains. The red shales of the Triassic formation of New Jersey 

 are in most cases consolidated sandy clay, but with one excep- 

 tion all those examined are of poor plasticity and very low 

 fusibility. 



