54 Fi—ELD CoLtuMBIAN MusEuM—GEo toey, VoL. IIL 
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 claystone 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 atalluncommon, ‘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. 
