436 SECONDARY LIMESTONE. 



in the sandstones, it would necessarily be ranked 

 among the modifications of a concretionary struc- 

 ture. 



The fibrous structure, already described in 

 the general remarks on the forms of rocks, occurs 

 in this limestone, yet it is not common. As in 

 the case of gypsum, it appears the result of crys- 

 tallization, and the fibres are always at right 

 angles to the plane of the strata. It is remarkable 

 that the same apparent stratum is sometimes 

 fibrous in one portion of its thickness and amor- 

 phous in the remainder ; or, if the two are really 

 distinct strata, they are not separable by mecha- 

 nical force. The fibres are generally both straight 

 and parallel, and they do not lose the latter cha- 

 racter even when slightly curved, as they some- 

 times are. In some rare instances, they present a 

 singular imperfectly ramified appearance, not to 

 be rendered intelligible by words, and very 

 much resembling, on a general view, the effect 

 that would be produced by an aggregate of or- 

 ganic remains. 



The small columnar, or large fibrous struc- 

 ture, also occurs in limestones, but still more 



