A CONTRIBUTION TO THE OOLITE PROBLEM 795 



the member at the Point Ann exposure only, and the samples here 

 described and figured are entirely from that locality. 



Microscopic examination of the rock shows it to consist of 

 imperfectly preserved siliceous oolite grains in a dolomitic matrix. 

 The history of the rock is briefly as follows: Subsequent to the 

 formation of the oolite, dolomitization set in, transforming the cal- 

 careous matrix completely, and many of the calcareous oolite grains 

 either wholly or in part, to dolomite. Alteration then ceased and 

 silicification of the unchanged, or only partly changed, oolite grains 

 ensued. The irregular areas of dolomite within the interiors and 

 the frayed-out borders of many of the silicified oolite grains are in 

 this way accounted for. The structure of grains which were com- 

 pletely dolomitized prior to silicification is almost entirely obliter- 

 ated, and these are often only with difficulty distinguished from the 

 matrix. 



The oolite grains range from o . 1 mm. to 1 . 13 mm. in diameter, 

 and when well preserved show, in addition to the concentric and 

 radial structure, minute sinuous, enwrapping fibers very similar to 

 the tubules which characterize the Girvanella type of calcareous 

 algae. A comparison of the microphotographs of the oolite grains 

 with that of Girvanella problematica Nicholson, described and figured 

 by Rothpletz, in his memoir entitled "Ueber Algen und Hydrozoen 

 im Silur von Gotland und Oesel," 1 will bring out this striking 

 similarity (Figs. 1-6). 



It should be recognized that the interwoven fibers of the oolite 

 have been partly obliterated by silicification. Doubtless these con- 

 sisted of hollow tubules filled with calcite, like those shown by Girva- 

 nella problematica prior to silicification. 



The fibers of the organism of the oolite have an average diameter 

 of 0.015 mm. which agrees very closely with the diameter of the 

 tubules of Girvanella problematica, which varies from 0.01 to 

 0.018 mm., according to Rothpletz. 



Typically the well-preserved oolite grains consist of an inner 

 structureless nucleus, followed by a narrow intermediate band 

 showing radial structure, and this again by an outer band bearing 



z Kungl. Svenska Velenskapsakademiens Handlingar, Band 43, No. 5 (1908), 

 PI. I, Fig. 1. 



