A. R. Horwood—Aragonite in Middle Lias, Leicestershire. 175 
forms a distinct band in section, being highly calcareous, and in 
character a crystalline limestone, consisting of at least 60 per cent. of 
carbonate of lime. From its composition it is thus practically pure 
calcite. Indeed, the organic contents are mainly calcite, and the 
inorganic residue consists of a small percentage of lime, with iron 
and a little detrital matter, though in this last respect it varies 
a good deal. 
Dr. R. Brauns’ has pointed out that calcareous deposits may be 
determined in their ultimate character by the temperature of the 
solutions from which they are precipitated, calcite being formed with 
its characteristic rhombohedra in a cold solution, or one in which 
there is a preponderance of alkaline silicates, whilst a warm solution, 
and one in which gypsum or strontianite occurs, is conducive to the 
formation of aragonite. ‘he amount of concentration of the solution 
influences the direction which the carbonate of lime will take, or its 
ultimate form as calcite or aragonite. Thus Credner? has found that 
a cold saturated solution produces calcite, a warm dilute solution 
aragonite. ‘These conclusions are borne out, moreover, by the 
physical relationship between the contents of this crinoidal limestone 
and the strata above. Thus above this horizon the beds are more 
ferruginous, less calcareous, often glauconitic, and of a distinctly more 
littoral type, the chief characteristics of the fauna being Gasteropods, 
Echinoderms, and Cephalopods. These beds would thus present, when 
unconsolidated and in a state of solution, a warmer temperature, and 
because they were further from the bottom of the basin or floor of the 
sea would be less concentrated with calcareous matter, but would 
contain more alkaline silicates. Thus aragonite would here replace the 
carbonate of lime, which does not amount to as much as 50 per cent. 
And this is what is actually the case, the shells being aragonite shells, 
though not confined to the zone or organisms mentioned and more 
constant at the horizon indicated. Cephalopods, Pinna, Amusium, and 
others all have the outer original aragonite layer removed, and are 
preserved as casts, with few exceptions. In the iron-ore beds the 
percentage of lime is 62:14 per cent., with ferric oxide 25-71 per cent. 
at Caythorpe and 30 to 33 per cent. at Tilton.2 But here and elsewhere 
the percentage of lime generally in the ore-bearing rock is very much 
lower, being about 13 per cent., whilst there is 29 per cent. of carbonic 
acid. In the crinoidal limestone band, however, the constituents are 
markedly different. Of lime there isa percentage of quite 60 per cent., 
and an absence of detrital matter or ferric oxide to an appreciable 
extent. This band, moreover, marks, as I have suggested, the 
culmination of pelagic and the inset of more littoral conditions. 
In this way it would naturally have been deposited as a more or less 
concentrated solution, because nearer the bottom into which all saline 
matter collects, and a colder solution from its abyssal character. 
Moreover, pure limestone usually denotes deposition far from land, 
and in this case the pure character may be due to absence of sandy or 
argillaceous matter and undisturbed deposition of lime, whilst the 
1 Chemische Mineralogie, 1896, p. 156. 
2 Neues Jahrb., 1871, p. 288. 
3 Grou. Mae., 1907, pp. 462-3. 
