336 PRINCIPLES OF STRATIGRAPHY 



however, is needed to distinguish such conglomerates from the basal 

 conglomerates marking a disconformity between the two layers. 

 The Cretacic chalk of western Europe has also been regarded as in 

 part made up of chemical precipitates. This refers especially to the 

 cement which unites the shells of foraminifera, etc. The chemical 

 part of the cement consists of microscopic calcite rhombohedrons 

 and less regular calcite grains, a large part of these originating from 

 the recrystallization of organic aragonite (Cayeux-9). Walther 

 regards the cementing material of the fine calcilutytes of the Soln- 

 hofen region as due to chemical precipitation (64:21^). 



Oolites and Pisolites of Chemical or Purely Hydrogenic Origin. 

 These have been described from a number of localities, occurring 

 both in recent formations and in older geological deposits. The fa- 

 miliar pisolite of the Carlsbad Springs in Bohemia is a typical ex- 

 ample, formed by the encrustation of minute particles of quartz or 

 feldspar, held in suspension by the rising waters, and turned in all 

 directions, so as to produce uniform encrustations. Gas and air 

 bubbles likewise may form the nuclei of such encrustations, which 

 in that case have a hollow center. The usual composition is CaCOg, 

 with a small quantity of FeCOg (Zirkel-zOi^^'^*; Hochstetter- 

 24:8^), but the mineral is neither calcite nor aragonite, but a third 

 modification "Ktypcit," convertible by heat into calcite. On the 

 shores of seas, the water of which holds much lime, particles of 

 broken shells, grains of igneous rocks, etc., are encrusted by CaCOg 

 forming a rock of typical oolitic appearance. On the shores of 

 the Island of Gran Canaria in the Canary Islands, between Las 

 Palmas and Isleta, encrustations of carbonate of lime are found, 

 according to L. von Buch, around fragments of broken molluscs, 

 and small sand grains of trachyte and basalt. This sand becomes 

 consolidated into a porous rock, which is quarried at low tide. The 

 water of the waves throughout the greater part of the year has a 

 temperature of more than 20° C, and seems to be abundantly able 

 to dissolve lime and redeposit it around the grains of sand, forming 

 a "Rogenstein." In the Mexican lagoons oolites are formed, ac- 

 cording to A'irlet d'Ooust, by the encrustation with lime of insect 

 eggs, and these oolites closely resemble those from the Jurassic. 



On the coast of Ascension Island recent oolites are formed, ac- 

 cording to Darwin, by the encrustation of rounded fragments of fos- 

 sils. Similarly, fragments of shells and grains of coral sand on the 

 north coast of Tahiti are coated with lime, according to Dana 

 (13 -J/-)' ^^^ transformed into oolites. Other examples are cited 

 from the coast of the Sinai peninsula at Wadi Deheese, and in 

 the Reede of Suez (Walther-59:6'6'^). These last two examples 



