762 T. C. BROWN — OOLITES AND OOLITIC TEXTURE 



moving currents of siliceous waters (probably hot springs), and that sub- 

 sequent to their deposition in layers the final cementing of the oolitic 

 spherules took place. He summarizes his observations as follows: 



"In conclusion, then, it may be said that siliceous oolite occurs predomi- 

 nantly in rocks of Upper Cambrian and Beekmantown age; that there is a 

 series of definite 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." ^® 



The writer has spent a great deal of time studying these siliceous 

 oolites, both in the field and in the laboratory. Most of the localities 

 mentioned by Ziegler have been examined very carefully, and several 

 other localities have been studied which are not mentioned in his or any 

 other paper. The majority of writers have considered the occurrence of 

 siliceous oolites a phenomenon confined to a limited area; but this is an 

 erroneous idea, for they are not only wide-spread in their distribution in 

 central Pennsylvania, but also occur among the calcareous oolites of the 

 eastern part of the State in the more highly metamorphosed beds of the 

 same horizon (see plate 27, figures 5 and 6). 



The typical State College siliceous oolites have already been well de- 

 scribed by both Hovey and Diller. The essential features are these: 

 The oolites are generally almost spherical in shape and range in size 

 from one to one and one-third millimeters in diameter. The majority 

 of them show a clear quartz sand grain nucleus, but some 'only show 

 granular quartz in the center. This does not, however, indicate that the 

 oolites without a sand grain necessarily lack a nucleus. While some 

 undoubtedly do lack nuclei, this fact has probably been exaggerated in 

 the descriptions thus far published. The accompanying figure (figure 1) 

 has been drawn to show how some of those grains which appear to lack a 

 nucleus may really have one which has not happened to be cut by the 

 plane of the section. It also serves to illustrate how the range in size 

 assigned to these oolites is undoubtedly greater than it really should be. 

 A double nucleus consisting of two sand grains within a single oolite can 

 now and then be seen (plate 28, figure 2). 



The quartz sand grain is very frequently enlarged by silica deposited 

 in optical continuity with it. This secondary enlargement may consist 

 of one or two zones, and when extensively developed may even show the 

 outlines of a quartz crystal partially developed (plate 28, figure 3). As 

 noted by Hovey, the first zone of secondary enlargement is always cloudy. 



»"V, Ziegler: Am. Jour. Sci., 4 ser., vol. xxiv, 1912, pp. 113-127. 



