764 T. C. BROWN OOLITES AKD OOLITIC TEXTURE 



ular form or as finely fibrous zones of chalcedony. The granular ma- 

 terial is quartz and forms a zone of varying width around the nucleus, or 

 if there is no nucleus apparent it fills the center of the grain. This zone 

 of granular quartz may fail completely and the fibrous chalcedony extend 

 all. the way to the nucleus, or it may make up practically the whole of 

 the oolite. 



In transmitted light the concentric rings of fibrous chalcedony fre- 

 quently show faint yellow outlines, as if minute quantities of iron oxide 

 had been deposited with the silica (plate 28, figure 1). "Occasionally 

 the nucleal quartz is enveloped in a thick, dense layer of oxide of iron, 

 and more rarely there are fijiely fibrous layers nearly midway between 

 the center and the circumference of the spherule.'' ^"^ 



Outside of the oolites proper, but concentric with them, though form- 

 ing a part of the matrix, occur incomplete bands of more coarsely fibrous 

 or finely crystalline material. Under very high powers of the microscope 

 these look like elongated colunmar crystals of quartz, placed perpen- 

 dicular to the surface of the oolites, with their free ends projecting into 

 the interspaces, which are rarely incompletely filled. These bands are 

 incomplete where the oolites approach each other closely. They evi- 

 dently belong to the matrix surrotmding the oolites, and the difference in 

 size and texture of the component fibers indicates clearly that they de- 

 veloped at a difi'erent time and under somewhat different conditions from 

 the fibrous material of the oolites proper. This fact has a direct bearing 

 on the origin of the siliceous oolites. 



Occasionally an oolite can be seen which has been fractured and the 

 two parts moved slightly relative to one another. This crack or fracture 

 has later been completely filled with crystalline quartz. 



Bellefoxte Type of siliceous Oolites 



In many of the beds of limestone and dolomite of the Beekmantown, 

 or Lower Ordovician,^^ siliceous oolites occur, as noted by Ziegler, either 

 in layers or as chert nodules. The State College t}'pe, with minor varia- 

 tions, is characteristic of the bedded layers wherever they occur, and they 

 can be obtained at various outcrops from Krumerine, near State College, 

 to Belief onte, a distance of nearly 12 miles. xA.s an example of the chert 

 nodule type of siliceous oolite, the writer has selected such a nodule from 



i" Dillon: Loc. cit., p. 96. 



38 E. O. Ulrich : The Canadian of ririch, including, from the top down, the Bellefonte 

 dolomite. Axeman limestone, Nittany dolomite, and Stonehenge limestone. Bull. Geol. 

 Soc. Am., vol. 20, 1911, pp. 657-659. 



