SALTER — FORMATION OF LIMESTONE BANDS. 



23 



lime, has served as a point of attraction for neighbouring small 

 quantities of lime, precisely as my friend suggests. And, really, the 

 analogy with flints, and not remotely so with metals, is much to the 

 point. 



I 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 : — 



rig. 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 — the projection of one 

 fitting opposite the recess of another, and the shale between 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 commonest 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 have 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 undei'gone 

 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 b can be determined except by calculation ; the bed 



