Ziegler — Siliceous Oolites of Central Pennsylvania. 125 



the oolites and their relation to the adjacent rocks have con- 

 vinced the author that no theory of origin will meet all the 

 conditions. 



The calcareous oolite so abundant in the Cambrian and Ordo- 

 vician rocks may be considered to be the result of direct depo- 

 sition from the lime-bearing ocean water. Most of the older 

 rocks are unfossiliferous and are conglomeratic in character. 

 Thus many of the lower Ordovician limestones are character- 

 ized by peculiar conglomerates of flat elongated limestone- 

 pebbles, irregularly mixed and shuffled together so as to sug- 

 gest the name " edge-wise " conglomerate to Stose.* As 

 Rothpletz suggests, most calcareous oolites are the result of 

 concretions of calcium carbonate about algse.f We may also 

 have direct deposition of calcium carbonate about grains of 

 limestone or quartz along the shore of partly enclosed seas4 

 This seems to be proven by the presence of sand-grain nuclei 

 in most cases, which suggest a deposition about rolling sand. 

 This hypothesis would postulate a shallow water condition for 

 this part of Pennsylvania, an assumption also supported by the 

 many shale partings in the limestone, which are commonly 

 ripple-marked. The prevalence of clastic rocks, such as the 

 edgewise conglomerate, the presence of cross bedding, the 

 rapid alternation in the character of the sediments, all seem to 

 confirm the idea that the sediments were laid down in shallow 

 water, while the occurrence of brachiopods such as lingulepis, 

 with their valves filled with oolites, seem to confirm the idea of 

 origin syngenetic with the limestone. 



In accounting for the origin of the oolitic concretions 

 described as type four, practically contradictory structures 

 must be taken into account; thus the entire make-up of the 

 quartz ring and nucleus and its analogous structure with the 

 oolitic limestone prove that they are a replacement of the latter. 

 The remarkable resemblance in size, shape, and the nuclei of 

 the oolites are the points in favor of this view. On the other 

 hand, the interior crystalline ring, the lines of contact between 

 this and the quartz, and the latter and the limestone, are points 

 just as strong which proves that calcite has replaced part of 

 the siliceous concretion, finding the cement the less resistant. 

 Thus the reaction apparently proceeded in one direction for a 

 time and then reversed due to a change in the physical or 

 chemical conditions, thus following the law of mass action. 

 We may represent the reactions as follows graphically : 



*U. S. G. S., Folio No. 170, and Geol. Mich., vii, p. 66. 

 fAm. Geol., x, p. 279. 

 % Geikie's Geology, p. 150. 



