SALTER — rOEMATlON OF LIMESTOI^E BA>'DS. 
23 
lime, has served as a point of attraction for neighbouring small 
quantities of lime, precisely as my friend suggests. And, really, tlie 
analogy with flints, and not remotely so with metals, is much to the 
point. 
1 had lately before my eye a thick series of nodular limestones in 
that little-known formation, the carboniferous slate. A rough draw- 
ing of it might be thus given : — 
Fig. 2. 
Here were thick beds, 28 feet and 25 feet respectively, of nodular 
limestone among shale, separated by a thick band of shale, h c, with- 
out limestone, or, rather, with only a uniform distribution in small 
proportion to the mass ; and a flat bed of lime in the middle, c. 
But the nodules were extremely regular — tlie projection of one 
fitting opposite the recess of another, and the shale betw^een follow- 
ing these sinuosities. These could never have been the lines of original 
deposition. Moreover fossils, though plentiful enough, did not form 
the mass of the limestones a. Here segregation is evident enough. 
It is the coi^imonest of all phenomena to meet with beds of 
lime-stone in which the lenticular form prevails, and to find such 
beds crowded with fossils. Here we liave the result of the two 
causes — plain deposition and subsequent segregation. The base 
Fig. 3. 
of the mountain limestone group gives excellent examples of both. 
The beds are one mass of shells and encrinites, and in them, dis- 
posed in every fanciful form, and yet in layers on the whole, are no- 
dules of chert. 
Some of the prettiest instances of the change of dimensions (I 
think it was Sorby who applied the term) in limestones occur when 
the rocks containing them are cleaved. In such cases, where cleav- 
age is well-developed, the limestone nodules will often form along the 
cleavage, i. e. the line of least resistance. In cases where great 
pressure has been exerted, the bed of limestone is often violently 
crumpled up, while the shale has quietly submitted to be squeezed, 
and "made no sign," though every layer of it must have undergone 
the same process. I have, in my sketch-book, a Devonian limestone 
bed, near Ilfracombe. Here are two lines of original deposit ; a and 
h were limestone bands, and are full of encrinites, but the thickness 
of neither a nor h can be determined except by calculation ; the bed 
