Maryland Geological Survey 177 



Of the three forms of glauconite differentiated by Collet, all three are 

 found in tlie.se sediments. The grains which in this paper have been 

 described as botryoidal are those called casts by Collet, that is, they are 

 believed to owe their form to their origin within the shells of Foraminifera. 

 A very few grains were noted that had the form of other small shells, bvtt 

 the shells were not further determined. The description of the products 

 shows that this form of glauconite occurs mainly in the medium and fine 

 sands, occurrences in the coarse sand having usually the appearance rather 

 of secondary agglomerations of smaller grains, while only few if any such 

 grains without signs of wear are found in the very fine sands. This dis- 

 tribution means a range of size pretty well within the limits of 0.3 mm. 

 to 0.9 mm. diameter. Collet 1 gives an upper limit of 1 mm. 



The second kind of grain defined by Collet is simply a grain showing no 

 trace of an original mould. To this category belong the rounded grains 

 which prevail in the very fine and finer portions of sediments with 

 primary glauconite, and which as reworked glauconite enter into other 

 beds. It is generally agreed that they are derived through the rounding 

 by attrition of the glauconite casts. 



To Collet's third type, the fragmentary glauconite, belongs what is 

 here called glauconite stain ; that is, the glauconite adhering like clay to 

 the outside or filling the fissures of mineral grains. 



Concerning the origin of glauconite, Collet's own conclusion that the 

 processes are still very little understood may be emphatically cited. But 

 the facts of observation at least give much evidence as to the conditions 

 under which it takes place. 



It is generally believed that a certain amount of organic matter is 

 essential to the process, but an excess of it seems, on the other hand, to 

 interfere. Collet 2 gives the formula, which appears to be generally 

 accepted, by which decomposition of organic matter precipitates FeS 

 (p. 171). As he explains, this FeS is believed to be capable of giving up 

 its iron directly to silicates to form iron silicates, but an excess of organic 

 matter interferes with the process and thus leads to the accumulation 



1 Collet, L. W., Op. cit, p. 133. 



2 Collet, L. W., Op. cit., pp. 169, 170. 



