760 T. C. BROWN OOLITES AlH) OOLITIC TEXTURE 



rock had formed, and that only an incipient change had taken place when 

 the replacement by silica was accomplished. 



Plate 27, figure 1, illustrates a calcareous oolite from eastern Penn- 

 sylvania, which presents peculiar features that can not yet be explained. 



Siliceous Oolites of Pennstlvanta 



Siliceous oolites were first discovered in the vicinity of State College, 

 Pennsylvania, and were described by d'Invilliers in his report on Center 

 County.^^ His description is as follows: 



"Along the southeast side of the Barrens, in the vicinity of State College, is 

 found a rock, so far as I know, peculiar to this locality. It has a general re- 

 semblance to oolitic limestone, but is composed of siliceous concretions, about 

 the size of mustard seeds, imbedded in a siliceous matrix. Its color is usually 

 white, though it is sometimes iron-stained. In other cases both concretions 

 and matrix consist of liornstone. Large quantities of this material may be 

 picked up from the surface in this region. 



"I found fragments of this same material near the Barrens in Walker town- 

 ship, near Snydertown." 



About six years later Wieland became interested in these oolites and 

 sent many samples to various institutions, both in this country and 

 abroad. He also described in a short paper their geographical occur- 

 rence.^^ Within the next few years three important papers appeared on 

 these oolites. The first was by Barbour and Torrey, and included an 

 illustrated description of the oolites and chemical analyses of different 

 samples, showing that they graded from almost pure silica to very highly 

 calcareous t3'pes. They concluded that the siliceous oolites were derived 

 from the calcareous forms. They recorded the presence of organic re- 

 mains as nuclei, but no such nuclei have been seen by any later investi- 

 gators.^^ 



In 1892 W. Bergt published a very good description of these oolites, 

 and pointed out the fact that silica does not now form by direct precipi- 

 tation at the surface in grains similar to these. Spherulitic forms do 

 occur in opal and chalcedony, but these can not be compared with the 

 Pennsylvania oolite. He concluded that these oolites were secondary, 

 due to the recrystallization of an original spherical grained opal or chal- 

 cedony.^^ 



The best microscopic description thus far published in this country is 



»E. V. d'Invilliers, Sec. Geol. Sur. Penna., vol. T4, 1884, p. 406. 



81 Mineralogists' Monthly, Nov., 1890. 



"^Am. Jour. Sci., 3d ser., vol. xi, 1890, pp. 246-249. 



» W. Bergt : Gesellschaft Isis in Dresden, Abh. 15, 1892, pp. 115-124. 



