A, R. Horwood—Aragonite in Middle Inas, Leicestershire. 177 
_ The very hard character of the crinoidal limestone band, made up 
so largely of crinoid stems, is due to the preservation of the con- 
stituents as calcite. And it is doubtless to a similar horizon that 
Dr. Sorby refers above, though there is no reason for assigning them 
in particular to Pentacrinus. In a section this band stands out some 
distance from the beds above and below, protecting the latter from 
denudation. In the quarry at Billesdon Coplow the position of this 
band is well shown, being indicated by a marked projection near the 
top of the section. The section shows, moreover, the position of 
the Transition-bed, the horizon of Hodiadema granulatum, and many 
Gasteropods. The same band is the horizon of the aragonite boulders 
at Tilton Hill. The hardness of the crinoidal band is indicated by its 
fresh surface when transported in glacial deposits, whereas the ferru- 
ginous marlstone is generally decomposed and weathered. The oolitic 
grains mentioned by Sorby as consisting of aragonite concretions occur 
in small glauconitic pockets of the Transition-bed, and upon faces of 
the jointed rock. These concretions are similar in character to those 
found—if the deposits are of carbonate of hme—in hot-water boilers, 
though aragonite alone is not found. Aragonite in a crystalline mass 
may depend upon the distribution of water in rocks, or the water-level. 
Thus W. Wallace’ found that it was only to be met with above the 
water-level in caverns in Cumberland. This is analogous to the 
distribution of aragonite as the replacing mineral in the shell-layer 
of fossil organisms. Messrs. Cornish & Kendall? found that shells in 
permeable strata had their aragonite layer dissolved out, whilst those 
protected by an impermeable layer above were preserved. To a less 
extent calcite is affected in the same way. 
In the littoral sandy, ferruginous, and readily permeable beds of 
the Middle Lias Marlstone a large percentage of the aragonite shells 
have their shell-layer dissolved away. The aragonite boulders above 
the crinoidal limestone band are similarly worn externally, probably 
by the action of water through percolation. In the first instance they 
must have been precipitated in a warm solution, and consequently at 
a period when the surface was elevated above the level at which the 
erinoidal band itself was formed. 
Aragonite is found in older calcareous rocks, chiefly Mountain Lime- 
stone or earlier rocks, in Cornwall, Devon, Dorset, Derby, Cumber- 
land, Durham, and Somerset; in Scotland at Lead Hills, Galloway, 
Seafield, Portsoy (Banff), Shetland Islands, and Orkneys; and in 
Ireland in Antrim, Derry, Down, Kerry, and MacGilligan. To these 
localities must now be added Tilton Hill,? Leicestershire. The best 
localities are Alston Moor and Cleator Moor, where there are hematite 
deposits. As a whole it is thus characteristic of the older rocks, 
when doubtless the mean thermal heat was greater than in later 
geological times. 
1 Quart. Journ. Geol. Soc., 1865, vol. xxi, pp. 413-21. 
2 «On the Mineralogical Constitution of Calcareous Organisms’’: Gxrox. Mae., 
1888, p. 66. 
3 Though so called this locality is close to Lowesby Station (G.N.R.), and must 
not be confounded with the cutting near Tilton Station (joint L.N.W.R. andG.N.R.). 
a locality more widely known than the Lowesby one. 
DECADE V.—VOL. VII.—NO. Iv. 12 
74 
