position have been recorded from the European Maastrichtian chalks (Jagt & Michels, Geologie en Mijnbouw, v.69, p.179, 1990) and from the Palaeocene of Madagascar (Lambert, Annales Gkoligiques Service des Mines, Madagascar, v.3, p. 1, 1993). All we have to do now is to look for shape

differences in echinoid populations and modifica- tions of the posterior ambulacra in the same speci- mens, to recognize sexual dimorphism and possi- ble brood care in other spatangoid lineages. Thus exposed, could this make echinoids seem more ‘human’?

Correspondence Carbonate cementation in the balance?

Sirs: This comment has been prompted by Niall Fleming’s article (Geology Today, v.9, p.223, 1993) on calcium carbonate cementation in sandstones. I was pleased that Fleming discussed and recom- mended for further reading some work I did on modern intertidal sediments in the Firth of Forth (Scottish Journal of Geology, v. 24, p.233, 1988). The reason 1 write is that both Fleming and I (in the original article) made mistakes in the chemical equations which depict the process by which the sandstones might have become cemented. It seemed that a comment about using chemical equations to depict geological reactions might be helpful, both in the specific case of the Firth of Forth sediments and in a more general sense for those readers who don’t often concern themselves with chemical equa- tions.

In my original article, I gave the following equa- tion for the oxidation of methane by aerobic bacte- ria:

4CH, + 90,+4HCO, + 6H,O. (1)

This equation is incorrect because it is unbalanced. Chemical equations should balance in two ways. First, when a chemical reaction takes place atoms are neither gained nor lost; thus in Equation (1) above, four atoms of carbon, 16 atoms of hydrogen and 18 atoms of oxygen should (and do) appear on each side of the equation. Second, the sum of the charges on each side of the reaction should be the same. In Equation (1) the right-hand side has four negative charges, while the left-hand side is neu- tral. The charges are clearly unbalanced.

A correct formulation of methane oxidation by aerobic bacteria would have been

CH, + 20,+CO, + 2H,O (2)

and this time both atoms and charges are balanced. The correct equation shows that carbon dioxide (CO,) - not bicarbonate (HCO,) - is the product of aerobic oxidation of methane. Equation (2) shows that aerobic oxidation of methane does not necessarily increase carbonate alkalinity (i.e. HCO; and C0:- concentrations). It is true that reaction between carbon dioxide and water could increase carbonate alkalinity:

CO, + H,OwHCO, + H+ (3)

but in the thin, near-surface oxic zone of tidal flats, much of the CO, would be lost by diffusion,

preventing buildup of HCO; and precipitation of calcite by the reaction

Ca2+ + HCO, wCaCO, + H’. (4)

It is therefore possible that anaerobic sulphate- reducing bacteria reduced the methane near the base of the sulphate-reduction zone using sulphate as the oxidant:

CH, + SO: +HCO;+HS + H,O. ( 5 )

This reaction allows rapid build up of HCO; m pore waters and supersaturation with respect to calcium carbonate, leading to calcite precipitation (Equation 4). It is this reaction (Equation 5 ) that Flemingmight have used to show best the anaerobic oxidation of methane, rather than his reaction 1, which is unbalanced with respect to both charges and atoms.

It is interesting to note that anaerobic oxidation of methane is not just responsible for cementation insandstones. Raiswell (Geology,v.16, p.614,1988) suggested that this process might account for the presence of many limestone concretions in marine shales (e.g. the concretionary horizons in the Lower Jurassic Jet Rock of North Yorkshire) and for the limestone bands in limestone-shale cycles (e.g. the famous Lower Jurassic Blue Lias sequences of Britain).

Finally, it is important to realize that chemical equations are usually simplifications of the actual chemical transformations that occur in nature. Chemical equations usually summarize a series of complicated reaction stages, illustrating a product we might reasonably expect to form (based on empirical or theoretical evidence) without neces- sarily depicting all the stages of reaction, or the complexity encountered in nature.

JULIAN ANDREWS School of Environmental Sciences

University of East Anglia Nonvich NR4 7TJ

de Luc’s Salisbury Plain in 1780 and 1810, and the rocks of Stonehenge

Sirs: In Current Archaeology (no. 134, 1993 - and see report in Geology Today, v.9, p. 163, 1993) John Darrah suggested that we have misinterpreted de Luc’s eighteenth-nineteenth-century description of Salisbury Plain in our paper on the Stonehenge bluestones (Thorpe and others, Proceedings of the

GEOLOGY TODAY May-June 1994195