766 T. C. BROWN — OOLITES AND OOLITIC TEXTURE 



silica-bearing solutions began to act on certain particular areas and com- 

 pletely changed the unaltered oolites to iibrons qnartz or chalcedony. 

 The little rhombic crystals already formed were more resistant than the 

 aragonite and were consequently unaffected. Later magnesian-bearing 

 solutions once more affected the stratum and all of the material not pre- 

 viously altered was then changed to dolomite. The silica-bearing solu- 

 tions did not stop when they completely altered the oolite grains within 

 the nodule, but began to replace the matrix around and among the 

 spheres. Thi^ matrix has in some interspaces been completely replaced, 

 while in other spaces a part of the material was left and later chan^red to 

 dolomite. 



A comparable course of events led to the formation of the State Col- 

 lege type of siliceous oolites. It differed from that outlined above in 

 three respects: (1) no magnesian waters were involved in the process; 

 (2) the oolites attacked by the silica-laden waters had in most cases 

 nuclei of quartz sand grains; (3) the silica-bearing solutions frequently 

 also contained iron. These differences caused the changes involved to 

 take place as follows : The oolites composed of unstable aragonite began 

 to gradually change to the more stable form calcite, just as in the organic 

 accumulations around coral reefs at the present time a part of the. ara- 

 gonite quickly changes to calcite. This change frequently started near 

 the foreign nucleus, but in many cases also near the outer margin of the 

 oolites; it gave rise to the minute rhombic crystals which can still be 

 seen under a high power of the microscope. Then silica-laden waters 

 began to percolate through this oolite stratum. Because of the molecular 

 influence or molecular afiBnity of the quartz sand grain nuclei, this silica 

 in many oolites began to replace the aragonite near the nuclear sand 

 grain and to secondarily enlarge that sand grain. The calcium carbonate 

 which had already changed to calcite was more stable and it remained as 

 minute rhombic crystals within the first zone of secondary enlargement, 

 giving to it the dusty appearance which ha,s already been noted. In many 

 cases this secondary enlargement continued until it passed beyond the 

 zone in which incipient calcite crystallization had taken place. In such 

 cases the aragonite more distant from the nuclear sand grain was com- 

 pletely replaced, giving rise to the clear outer zone of secondary enlarge- 

 ment, and this zone frequently develops partial to more or less perfect 

 crystal boundaries of quartz. The outer zones, for reasons which it is 

 not easy to explain, are generally composed of granular or finely fibrous 

 quartz or chalcedony. The interspaces, as in the previous case, are gen- 

 erally filled with somewhat coarser fibrous quartz, all of the fibers being 

 placed perpendicular to the circumference of the original oolite when 



