CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 21 



result of secondary changes affecting the limestone. It is of frequent 

 occurrence in the calcareous material which occupies the interior of 

 fossils, such as the shells of Mollusca or the crusts of Trilobites, in which 

 case the columns radiate from the surface of attachment. In other cases, 

 it occupies irregular winding spaces in the rock, when it exhibits a con- 

 centrically banded structure, indicating its formation in successive layers, 

 while the constituent fibres radiate inwards in all directions from the 

 bounding surfaces of the mass. Under ordinary conditions, this peculiar 

 fibro-crystalline structure can only be regarded as purely inorganic. In 

 some instances, however, an apparently identical structure is produced 

 by the partial crystallisation of organic remains, such as the calcareous 

 skeletons of Stromatoporoids or Corals. It would appear that the so- 

 called Stromatactis of Monsieur E. Dupont, which plays a very important 

 part in the formation of some of the Devonian limestones of Belgium, 

 is really of the nature of the above-mentioned fibro-crystalline masses, 

 being partly of inorganic origin, and probably in part the result of sec- 

 ondary change in suitable fossils, such as Stromatoporoids. 



Any of the great groups of Invertebrates in which a calcareous 

 skeleton is produced may take a more or less prominent part in the 

 formation of a limestone ; and the principal facts connected with 

 this subject will be dealt with in greater detail in treating of each 

 group of animals separately. It may be well, however, to indicate 

 here, in the briefest manner, the chief groups of organisms, whether 

 animal or vegetable, which may be considered as pre-eminently 

 makers of limestones. As regards animals, a very important place 

 must be assigned to the Foram- 

 inifera, a group of the Protozoa in 

 which a calcareous shell is com- 

 monly developed. So far as the 

 older Palaeozoic limestones (Or- 

 dovician, Silurian, and Devonian) 

 are concerned, it is noteworthy 

 that in very few instances, so far as 

 yet known, do the tests of Foram- 

 inifera constitute a prominent con- 

 stituent of the rock. In all the 

 later formations, however, begin- 

 ning with the Carboniferous, we 

 meet with limestones which are 

 more or less highly charged with 

 the calcareous tests of these min- 

 ute organisms, sometimes in such 



numbers that the rock becomes what may be properly called a 

 " Foraminiferal limestone" (fig. 6). Of this nature are the " Sao 

 cammina limestone " of the North of England, the " Endothyra 

 limestone " of North America, and the " Fusulina limestone " of 

 Russia, all of which are of Carboniferous age. Of the Foram- 



Fig. 6. — Section of Carboniferous lime- 

 stone from Spergen Hill, Indiana, U.S., 

 showing numerous large-sized Foraminifera 

 (Endot/iyra) and a few oolitic grains, mag- 

 nified. (Original.) 



