ORIGIN OF LIMESTOM>. 31 
Mr. Allan Dick, who has kindly examined ti specimen of 
Gotham Marble, tells me that dark portions of the stone are 
not due to the presence of manganese- or iron-ores, but are 
probably due to carbonaceous matter. 
The probable sedimentary origin of some of the Triassic lime- 
stones, has been suggested by Mr. Horace T. Brown and myself.* 
An analysis of a limestone from Litton, near Chewton Mendip, 
was made by Mr. Brown, and subsequently he obtained specimens 
of similar rocks from Chew Stoke, and had a series of sections cut 
for microscopic examination. Referring to the sections of these 
rocks, Dr. Sorby wrote, "There appears to me little or no doubt 
that a large part of the material has been derived from the Car- 
boniferous Limestone * * * Some of the grains are clearly 
fragments of an older limestone, which was not only in some 
places oolitic, but which I think also shows clearly the presence 
of Foraminifera like those in Carb. Limestone. There are also 
fragments that might be derived from such a rock, as broken 
joints of Encrinites, and probably of Coral and Shell. On the 
whole it is an excellent case of what I have often thought must 
occur in some cases, viz., a mud derived from a limestone rock."f 
With regard to the chemical aspect of the subject more information is 
desirable. The small amount of carbonate of lime held in sea-water (the 
per-centage in the total saline matter being 0'345), has led to the belief that 
much of this material, brought down by rivers, is converted into sulphate of 
lime, the per-centage of which among the saline ingredients is 3 '6. From 
the sulphate of lime it is maintained that marine organisms derive the 
substance they need for their hard parts, while their ultimate decomposition 
brings about a precipitate of carbonate of lime. These conclusions, however, 
do not affect the question of the possible sedimentary origin of some calcareous 
deposits. 
Prof. Dittmar has pointed out that alkaline sea-water, if given sufficient 
time, will take up carbonate of lime in addition to the matter it already con- 
tains. The solvent powers of sea-water, due to the presence of free carbonic 
acid, do not appear to be very great, for although this gas is obtained from the 
decay of marine plants and animals, from the atmosphere, and possibly from 
submarine volcanic sources, its presence in a free state in sea-water is the 
exception. J Mr. W. S.Anderson says, "The soluble action of sea-water on 
amorphous carbonate of lime has nothing to do with carbonic acid. An 
artificial sea water, free from carbonic acid and carbonates of any kind, will 
dissolve up quite as much. It is distinctly confined to the soluble action of 
the salts present." 
Prof. Sollas has suggested that " rivers sometimes bear to the 
sea considerable quantities of undissolved calcareous matter 
derived from the formations through which they flow."|| This 
is a matter that requires confirmation, for it is known that car- 
bonate of lime is more readily soluble ir. fresh-water than in sea- 
* Geol. Eng. and Wales, Ed. 2, p. 233. 
f Letter to Mr. H. T. Brown. See also Sorby, Address to Geol. Soc., 1879 ; and 
Prestwich, Geology, vol. i. p. 110, vol. ii. p. 318. 
Report on the voyage of H.M.S. " Challenger" ; Physics and Chemistry, vol. i. 
pp. 203-222; see also J. Murray, Nature, Feb. 28, 1889, p. 426; Ibid., June 12, 
1890, p. 165 ; and A. Agassiz, Mem. Amer. Aciid. Arts & Sc., vol. xi., pp. 114, 125, Sec. 
R. Irvine and G. S. Woodhead, Proc. R. S. Edin., vol. xv. p. 308, vol. xvi. p. 324 ; 
R. Irvine and G. Young, Ibid., vol. xv. p. 316; and J. G. Goodchild, Geol. Mag. 
1890, p. 76, and Trans. Cumberland and Westmorland Assoc., No. xvi., 1891. 
Proc. R. S. Edin. vol. xvi. p. 322. Sec also W. G. Reid, Ibid., vol. xv. p. 151. 
|| Quart. Journ. Geol. Soc., vol. xxxix. p. 614. 
