LIMESTONES. 319 



"In thin sections under the microscope the unbroken shells of Foram- 

 inifera are very conspicuous. They lie in close proximity to each 

 other, and their inflated chambers, filled with crystals of calcite, some- 

 times occupy more than one third the area of the entire field. It is 

 certain that more than one fourth, and in some instances more than 

 one third, of the volume of the chalk is composed of foraminiferal shells 

 still practically entire. The matrix in which the shells are embedded 

 is made up of a variety of objects, the most numerous and the most 

 conspicuous under proper amplification being the circular or elliptical 

 calcareous discs known as coccoliths. The small rodlike bodies to 

 which the name rhabdoliths has been applied are not very common, 

 although their pressure is easily detected with a moderately high-power 

 objective. Mingled with coccoliths and rhabdoliths are numerous 

 fragments that are evidently the debris resulting from comminution of 

 foraminiferal shells. When the chalk is treated with acid there remains 

 a small amount of insoluble matter consisting of clay, fine grains of 

 quartz sand, minute pebbles not exceeding 5 millimeters in diameter, 

 and a very few internal casts of the chambers of Foraminifera. Nearly 

 all the foraminiferal shells have the chambers filled with calcite; a few 

 have these cavities still empty; but in a small number of cases the 

 chambers were filled with an opaque, insoluble mineral, probably silica 

 deeply stained with iron oxide, that remains as perfect internal casts 

 after the shell has been dissolved in acid. The amount and compo- 

 sition of the residuum varies with the purity of the chalk. In some 

 samples it scarcely exceeds 1 per cent, in others it is equal to 10 per 

 cent." 



Chalk was probably deposited in deep, quiet water little affected 

 by debris from the land. At present material of exactly similar type 

 is being formed in the deeper portions of the North Atlantic and other 

 oceanic basins. 



Distribution of chalk and soft limestones. Both the true chalks 

 and the other soft limestones here considered are of comparatively recent 

 geologic age, occurring only in Cretaceous or Tertiary rocks. There 

 is also a certain geographic unity apparent, for both types occur only 

 along the Atlantic and Gulf coasts and in the Western States. For 

 detailed information regarding the distribution of these rocks reference 

 should be made to the papers and reports listed on page 322. In the 

 present place only a summary can be given covering the more impor- 

 tant features of the subject. 



The true chalks occur only in formations of Cretaceous age in certain 

 Southern and Western States. The principal chalk deposits available 



