7MS T. C. BROWN OOLITES AND OOLITIC TEXTURE 



formed ardund hot springs, but are widely distributed at many horizons 

 in this Lower Ordovician limestone. Chert bands and even bands of 

 crystalline quartz resulting from the replacement of the original lime- 

 stone and containing minute rhombic crystals similar to those within 

 the oolites can be found. Where the limestones have been cracked and 

 shattered during folding, quartz stringers and quartz veins can be found. 

 An almost exact parallel to the course of events here outlined for the 

 silicification of these oolites has led to the preservation of fossils at a 

 slightly higher horizon in a magnesian limestone bed which has been 

 quarried near Henderson's Station, Montgomery County, Pennsylvania. 

 The dynamic forces affecting these limestone beds have completely de- 

 stroyed the fossils except where they have been preserved by silicification. • 

 In this case the fossils, Maclurea and Rapliistoma, possessed a shell more 

 resistant than the surrounding matrix. The silica-bearing solutions 

 therefore attacked the matrix, removing that and depositing quartz 

 about the fossil shells. This quartz gave rise to quartz crystals, with 

 their long axes perpendicular to the surface of attachment, and they 

 continued to grow until they filled the cavities within the shells, and 

 when the shells were close together the interspaces among the adjacent 

 shells also. This produces a result that is apparently exactly comparable 

 to the filling of the interspaces among the oolites, except that in the 

 former case the quartz crystals frequently attained a length of an eighth 

 to a quarter of an inch, while in the latter they rarely attained the same 

 fraction of a millimeter in length. In the original Henderson Station 

 fossils the shells were evidently of calcite and more resistant than the 

 matrix. They have, however, been removed by circulating waters and 

 are now indicated only by the hollow molds within the quartz casing. 



Clinton oolitic Iron Ore 



The Clinton oolitic iron ores are so widely distributed through the 

 Eastern States from New York to Alabama and westward to Wisconsin 

 and they occur within such narrow limits vertically in the geological 

 column that they early attracted attention and have frequently been dis- 

 cussed. The Clinton ores are generally divided into two classes — the 

 fossiliferous ores and the oolitic ores. The oolitic character is, however, 

 the most constant feature and only the oolites are to be considered here. 

 In describing the N"ew York ores, K'ewland and Hartnagel say that one 

 of the most characteristic features is ^*^the almost universal presence of 

 oolitic grains in the ores, even those which are apparently of purely fos- 

 siliferous nature." ^^ Oolitic grains are almost always present in the 



•» N. Y. state Museum Bull. 123, p. 51. 



