Ziegler — Siliceous Oolites of Central Pennsylvania. 127 



pressiire. A few of the layers are highly cemented and form 

 quartzite. These show secondary enlargement in many cases 

 and a cement of fibrous chalcedony. Angular fragments of 

 limestone are often included in the sandstone layers and seams, 

 and cracks in the limestone are often filled with the sand. 

 The character of these cracks shows that they were joint cracks 

 formed subsequent to the hardening of the limestone, and that 

 at this time the sandstone was not yet cemented, and hence was 

 squeezed into these cracks to form the miniature dikes. Sub- 

 sequent to their formation silica-laden water percolated through 

 these rocks, finding their readiest passage in the porous sand- 

 stone, at the same time depositing their silica about the quartz 

 grains and forming the layers of siliceous oolite. As is evident 

 in the case of the Krumrine oolite, calcareous oolites were also 

 replaced by silica to some extent, probably by the same solu- 

 tions which formed the siliceous oolite on Spring Creek, and a 

 comparison with fig. 6 will show the close analogy between 

 them. In order to have perfect oolites, such as are present in 

 these rocks, a certain amount of free motion is usually consid- 

 ered requisite, which would of course be a factor opposing the 

 above assumption. If we assume, however, that some of the 

 sandstone consisted of grains already concentrically enlarged, 

 shifted back and forth by water currents, and then deposited 

 in layers with the limestone, we overcome this obstacle. Sub- 

 sequent to their deposition in strata the tinal cementing of 

 the oolitic spherules took place. Many of the oolite layers 

 have enclosed in them angular fragments of different oolites 

 (see figs. 3 and 8), also pebbles of limestone, and frequently 

 chert nodules. That contemporaneous erosion was active, and 

 that this was a period of oscillations between the land and the 

 sea, has already been shown. The solutions were in all proba- 

 bility hot springs, as Wieland claims, and were located on the 

 shore of the Calciferous sea. The retreats of the sea gave the 

 opportunity for the hot, silica-laden waters to replace some of 

 the layers of oolitic limestone, but what is more important, to 

 form by direct deposition about the sand grains the siliceous 

 oolites. As a result of the unstable coast line some of the 

 oolite layers were eroded and their fragments were re-depos- 

 ited in some cases in lime-muds, in other cases in other oolite 

 horizons. Siliceous spherules of oolite also occur through 

 many of the associated limestones and so strengthen this view. 

 Summary. — In conclusion, then, it may be said that siliceous 

 oolite occurs predominantly in rocks of Upper Cambrian and 

 Beekmantown age ; that there is a series of detinite layers, a 

 conservative estimate being at least twenty ; that some of 

 the layers are the result of replacement of oolitic limestones, 

 but that the majority are the result of direct deposition of silica 

 from hot solutions about pure quartz sand. 



