
70 STRUCTURAL AND FIELD GEOLOGY 
forms of marine life—more or less reduced to the condition 
of a fine meal or flour. Odtte is similarly composed of 
organic débris, but is characterised by the odlitic structure 
already described as occurring in certain chemically formed 
calcareous deposits. In thin sections seen under the micro- 
scope the spherules show a concentric and radiated structure 
—the latter, however, being sometimes wanting. Similar 
spherules have been observed forming not only in mineral 
springs such as those of Carlsbad (Sfrudelstezn), but in 
shallow water in the Great Salt Lake of Utah, and on the 
coral beaches of the Bahamas. They obviously owe their 
origin to deposition of calcareous matter on particles of sand 
which are kept in motion so as to become more or less 
equally encrusted. Shell-marl is an earthy aggregate of 
shells (most frequently of freshwater origin), with a larger or 
smaller proportion of argillaceous matter. It often passes 
into Lacustrine Limestone, which is usually a fine-grained, 
dull white or grey rock—sometimes earthy, sometimes 
compact. 
Limestones, which are composed conspicuously of the 
débris of crinoids, corals, or shells, as the more prominent 
ingredients of the rock, are known respectively as Cvznozdal, 
Coral, and Shelly limestones (see Plate XX. 2). Occasion- 
ally, the organic structure of such rocks has been obscured or 
even entirely effaced by subsequent molecular changes—the 
mass becoming crystalline. Shelly limestones often acquire 
special names, according to the relative abundance of some 
particular sheli, as N«usm«utlite-, Hippurite-, Ammonite-, 
Gryphea-limestone, etc. Common Limestone is usually grey 
or blue, and fine-grained to compact. In many common lime- 
stones the organic structure is only revealed in thin sections 
under the microscope. On the weathered faces of such rocks, 
however, fossil remains may often be readily detected. There 
are many varieties of common limestone characterised by 
the presence of certain impurities and admixtures. Amongst 
these may be mentioned Cornstone, a highly calcareous sand- 
stone or arenaceous limestone, sufficiently rich in calcium 
carbonate to be burnt for lime, when a better rock is not 
available. Cesent-stone is a dull argillaceous and sometimes 
ferruginous limestone, often occurring in thin beds and layers 

