PENNSYLVANIA SILICEOUS OOLITES 765 



the upper part of the Beekmantown ( Belief onte dolomite of Ulrich) for 

 description. 



This is from the same horizon and is an inclusion in the dolomitic 

 oolite bed already described (see page 759). The oolites in this bed are 

 very uniform in size and much smaller than those of the State College 

 type. A series of measurements indicates that the range is between .45 

 mm. and .55 mm. in diameter. Probably the latter measurement is 

 nearer the average, because the sections giving the smaller values may 

 not have passed through the centers of the oolites. Neither the siliceous 

 oolites nor the associated dolomitic forms have nuclei of sand grains, nor 

 do they show any evidence of ever having had such nuclei. The majority 

 of the siliceous spherules are aggregated into a single mass or nodule, 

 but a few individual siliceous spherules occur completely surrounded by 

 dolomitic grains and at some little distance from the general nodule. 

 Small rhombic crystals occur within the majority of the siliceous oolites. 

 When well within the oolite, these rhombic crystals show considerable 

 variation in size and hold no definite position in the oolite, although 

 they are generally near the center. Some of the siliceous oolites show 

 one, or occasionally two, rings of these rhombic crystals near their outer 

 margin. The oolite proper is wholly made up of cr3rptocrystalline quartz 

 in component parts too small to be resolved by the microscope. These 

 oolites, however, like those from the State College locality, are often en- 

 larged by incomplete zones of definite quartz fibers radiating from their 

 outer margins into the interspaces among the grains. These are clearly 

 secondary, for they fail where the oolites were originally in contact. 

 Some of the interspaces among the siliceous oolites are completely filled 

 with quartz; others are still filled with crystalline dolomite. In the 

 wholly dolomitic part of the specimen a zone of minute rhombic dolo- 

 mite crystals, similar to those in the outer margin of the siliceous spher- 

 ules, surrounds nearly every oolite (see plate 27, figure 3). 



In a thin section cut from the boundary between the siliceous nodule 

 and the adjacent dolomitic rock the oolites on either side of this bound- 

 ary are seen to be identical in size and shape ; they each have the zone of 

 minute rhombic crystals of dolomite near the outer margin, and they 

 differ only in that the siliceous oolites consist largely of fibrous quartz or 

 chalcedony, while the others are made up of granular dolomite. These 

 oolites were evidently derived from similar original spherules, and it is 

 safe to assume that these originals consisted of calcium carbonate in the 

 mineral form aragonite. They were evidently first attacked by magne- 

 sian water§ which began to develop in the outer zones of these spherules, 

 and sometimes deep within them, the minute rhombic crystals. Then 



