Ziegler — Siliceous Oolites of Central Pennsylvania. 119 



many grains are angular. At times the nucleus is made up of 

 several grains of quartz. The quartz enlargement of the sand- 

 grain nuclei is fibrous chalcedony, and also the fine chert mosaic 

 deposited in concentric rings, as many as twelve being faintly 

 visible. A few of the quartzes show resorption, presumably 

 by solution which subsequently served to enlarge them, partly 

 replacing the original grain with chalcedony. The same pro- 

 cess is indicated by the absence of definite outlines in some 

 sand grains and their tendency to pass by insensible gradations 

 into the fibrous chalcedony. Many of the oolites are entirely 

 chert or chalcedony, and have completely replaced the nucleus; 

 but even in this case, under crossed nicols the outline of the 

 quartz grain which served as a center of deposition is faintly 

 visible. The quartz-grains appear to be igneous in origin and 

 carry fine trails of dust, and very small inclusions of a mineral 



Fig. 4. Fig. 5. 



Fig. 4. Filmore type of siliceous oolite. x 72. 



Fig. 5. Krainrine type, at contact between calcareous and siliceous 

 oolite, x 32. 



apparently biotite. Fibrous chalcedony also fills most of the 

 interstices between the oolites, and is usually in indistinct bands 

 parallel to their general outline. Some of the quartzes show 

 a tendency toward secondary enlargement. Very little calcite 

 is present, and that seems to be a secondary infiltration subse- 

 quent to the oolite formation. The sand nuclei vary in size 

 from minute fragments up to •62 mm in diameter. The longer 

 diameter often exceeds the shorter four or five times. 



Type Two : the Altro phase. — The siliceous oolite occurring 

 in the vicinity of Altro is the handsomest in Center County. 

 It is practically pure quartzite in composition, as shown by the 

 following analysis by Wieland :* 



* This Journal (4), iv, p. 262. 



