284 PRINCIPLES OF STRATIGRAPHY 



The giant pisoliths * of Zepce, Bosnia, range from 2 to 13.5 centi- 

 meters in diameter, but ordinary ooliths do not range much above 

 1.5 mm. In the Bosnian examples the kernal is magnesite and the 

 surrounding shells dolomite. Both concentric and radial structure 

 is shown in these and they are believed to have been formed in hot 

 springs, similar to the pisoliths of the Carlsbad Sprudel. 



True ooliths and pisoHths are precipitations (hydrogenic or bio- 

 genic) and their spherical form is an original structure. Rocks 

 of clastic or worn calcite grains or fragments of shells, etc., may 

 resemble ooliths, and are indeed often mistaken for them. To such 

 Bornemann has applied the term pseudo-ooliths, and they belong 

 to the category of exogenetic rocks, unless, indeed, they are subse- 

 quently enlarged by deposition of lime about them. Concretions 

 may also assume the form of ooliths or pisoliths, but they are 

 secondary or diagenetic structures. Three modes of origin have 

 been most widely discussed by geologists and one or the other has 

 frequently been regarded as the sole mode. These are : i. Chemical, 

 or hydrogenic ; 2. animal, or zoogenic ; and, 3. plant, or phytogenic. 

 Each of these will be discussed under its appropriate heading in 

 succeeding chapters. 



II. The Exogenetic, or Clastic Rocks 



In discussing next the clastic rocks, the agent active in the 

 production of their present characteristics will be considered as of 

 primary importance in making the larger divisions. It is not always 

 possible to determine what was the cause of the clastic condition 

 of a given rock, since other agents subsequently active have pro- 

 duced those features which give the rock its most characteristic 

 aspects. Thus a sand mass may owe its origin in part to atmos- 

 pheric disintegration and in part to the mechanical activity of the 

 water. Its final form, however, may be given by aeolian action, 

 the mass becoming finally by diagenism a consolidated sand-dune. 

 It will, however, be observed that the agent last active, and there- 

 fore the one whose characteristics were most strongly impressed 

 upon the mass, i. e., the wind, is not only responsible for the form 

 and structure which the deposit eventually takes on, but also, in part 

 at least, modifies the original form of the component grains. Thus 

 without overlooking the claims of the other agents those of the wind 

 may be considered as greatest and the rock is therefore placed under 



the division of wind-formed elastics, or anemoclastics. 



• 



* These should be classed as hydrospherytes. 



