54 Field Columbian Museum — Geology, Vol. III. 



clays have a true specific gravity between 2.62 and 2.65. This clay 

 persists unchanged throughout the substance of all claystones formed 

 in it. Any pebble or other foreign substance in the clay is enclosed 

 by and made a part of any clay stone that forms in the proper position. 

 With the exception of quartz any pebble likely to be encountered in 

 concretion-bearing beds is considerably heavier than the surrounding 

 clay. Bits of shell, frequently encountered in claystones from some 

 localities, render the concretion in which they occur heavier than 

 normal. Rock flour clays may, and frequently do, contain pulverized 

 minerals of many species, practically all of which are heavier than 

 the normal quartz and kaolin. Spots and seams stained with iron 

 oxides, segregations of magnetic iron sand, pulverized hornblende, 

 etc., are not at all uncommon. The cement of a claystone is, so far as 

 known, essentially calcium carbonate. Usually it is somewhat 

 magnesian and occasionally ferriferous. In either case the specific 

 gravity of the concretion is increased. Fortuitous variations in 

 composition and structure therefore commonly increase the specific 

 gravity. It is astonishing that in bodies apparently subject io 

 purely fortuitous changes so many and so great, this change of 

 density with form should not be entirely masked. That it is not so 

 masked, suggests that there are only narrow limits of structure and 

 quality of clay and cement within which the formation of these 

 concretions is possible. 



