written contributions to the discussion of a paper previously taken as read

WRI'ITEN CONTRIBUTIONS TO THE DISCUSSION OF A PAPER PREVIOUSLY TAKEN AS READ The Editor Dear Sir, I would like to comment on a number of points in Dr. Tresise's (1960) interesting paper on the Wessex Upper Greensand. Tresise suggests well-oxygenated waters are unsuitable for the formation of glauconite. Studies of recent glauconitic sediments indicate, however, that glauconite forms under conditions of good oxygenation, turbulence and slow deposition on continental shelves, swells or banks (van Andel & Postma, 1954; Collet, 1905; Murray & Renard, 1891). However, local semi-reducing conditions provided, perhaps, by decomposing organic matter seem to be necessary, because about 12 % of the total iron in glauconite is in the ferrous state (Burst, 1958). Tresise's extension of the term 'rnetacolloidal' to cover structures seems most undesirable; Rogers (1917) advocated using Wherry's (1914) term 'metacolloidal' -signifying microcrystalline substances of colloidal origin-to avoid loose usage of the terms 'amorphous' and 'colloidal'. Finally, I am unable to agree that the bulk of the glauconite is of colloidal origin. Much of the available evidence suggests an origin by alteration of other silicates, mainly clay minerals and biotite. Neither the random aggregate structure nor desiccation cracks in glauconite necessarily imply an origin involving a colloidal or gel stage. REFERENCES VAN ANDEL, Tj. & H. POSTMA. 1954. Recent Sediments of the Gulf of Paria. Reports of the Orinoko Shelf Expedition, 1. BURST, J. F. 1958. Mineral Heterogeneity in Glauconite Pellets. Amer. Min., 43, May-June. COLLET, L. W. 1905. Les Concretions phosphatees de l'Agulhas Bank. Proc. roy. Soc. Edinb., 25, 862-93. MURRAY, J. & A. F. RENARD. 1891. Scientific Results, H.M.S. 'Challenger'. Deep Sea Deposits. ROGERS, A. F. 1917. A Review of Amorphous Minerals. J. Geol., 25, 515-35. TRESISE, G. R. 1960. Aspects of the Lithology of the Wessex Upper Greensand. Proc. Geol. Ass.• Lond., 71, 316-39. WHERRY. 1914. J. Wash. Acad. sa.. 4, 112. H. H. ZUMPE, Geology Department, Chelsea College of Science and Technology. The Editor Dear Sir, There are several aspects of Dr. Tresise's (1960) interesting paper with which I must disagree. It is incorrect to assume that the base of the Chalk can be taken as approximating to a time-plane (p. 334), for over most of Dorset the basement bed is of Middle Cenomanian age, and not Lower Cenomanian as elsewhere, and so is also diachronous. Because of this, the Upper Greensand over parts of Dorset was exposed longer and suffered a greater amount of erosion. 265

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Page 1: Written contributions to the discussion of a paper previously taken as read

WRI'ITEN CONTRIBUTIONS TO THE DISCUSSIONOF A PAPER PREVIOUSLY TAKEN AS READ

The EditorDear Sir,I would like to comment on a number of points in Dr. Tresise's (1960) interestingpaper on the Wessex Upper Greensand.

Tresise suggests well-oxygenated waters are unsuitable for the formation ofglauconite. Studies of recent glauconitic sediments indicate, however, thatglauconite forms under conditions of good oxygenation, turbulence and slowdeposition on continental shelves, swells or banks (van Andel & Postma, 1954;Collet, 1905; Murray & Renard, 1891). However, local semi-reducing conditionsprovided, perhaps, by decomposing organic matter seem to be necessary, becauseabout 12% of the total iron in glauconite is in the ferrous state (Burst, 1958).

Tresise's extension of the term 'rnetacolloidal' to cover structures seems mostundesirable; Rogers (1917) advocated using Wherry's (1914) term 'metacolloidal'-signifying microcrystalline substances of colloidal origin-to avoid loose usageof the terms 'amorphous' and 'colloidal'.

Finally, I am unable to agree that the bulk of the glauconite is of colloidalorigin. Much of the available evidence suggests an origin by alteration of othersilicates, mainly clay minerals and biotite. Neither the random aggregate structurenor desiccation cracks in glauconite necessarily imply an origin involving acolloidal or gel stage.

REFERENCESVAN ANDEL, Tj. & H. POSTMA. 1954. Recent Sediments of the Gulf ofParia. Reports of

the Orinoko Shelf Expedition, 1.BURST, J. F. 1958. Mineral Heterogeneity in Glauconite Pellets. Amer. Min., 43,

May-June.COLLET, L. W. 1905. Les Concretions phosphatees de l'Agulhas Bank. Proc. roy. Soc.

Edinb., 25, 862-93.MURRAY, J. & A. F. RENARD. 1891. Scientific Results, H.M.S. 'Challenger'. Deep Sea

Deposits.ROGERS, A. F. 1917. A Review of Amorphous Minerals. J. Geol., 25, 515-35.TRESISE, G. R. 1960.Aspects of the Lithology of the Wessex Upper Greensand. Proc.

Geol. Ass.• Lond., 71, 316-39.WHERRY. 1914.J. Wash. Acad. sa.. 4, 112.

H. H. ZUMPE,

Geology Department,Chelsea College of Science and Technology.

The EditorDear Sir,There are several aspects of Dr. Tresise's (1960) interesting paper with which Imust disagree.

It is incorrect to assume that the base of the Chalk can be taken as approximating to a time-plane (p. 334), for over most of Dorset the basement bed is of MiddleCenomanian age, and not Lower Cenomanian as elsewhere, and so is alsodiachronous. Because of this, the Upper Greensand over parts of Dorset wasexposed longer and suffered a greater amount of erosion.

265

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