Prof. Sedgwick on the Structure of large Mineral Masses. 469 



some measure, why the sih'ca of a chalk flint has struck upon the fibres of 

 bodies allied to sponge ; and why the outer substance of the Echinodermata 

 is so frequently changed into carbonate of lime. But how do we explain the 

 fact, that trunks of trees imbedded in siliceous sand are in some places mi- 

 neralized by carbonate of lime, while other trunks imbedded in limestone are 

 mineralized by silica ? that in one limestone country, shells are all petrified 

 by carbonate of lime; while in another, shells of the same species, and even 

 the delicate portions of the internal animal structure (e. g-. the internal spire 

 of a Spirifer), are petrified by silica? To explain pha^nomena such as these, 

 or even to classify them rationally, would, I believe, require very extended 

 observations. But, whatever might be the answers given to such questions, 

 the reasoning of this paper would remain unaffected. For, whatever be the 

 mineral composition of a laminated nodule, or of a petrifaction imbedded in 

 any secondary rock, it is equally certain that such rocks must have undergone 

 a great chemical change since the time they were first deposited. 



§ 3. Cleavage, Rocks of Slati/ Structure, Sfc. 



The most striking modifications of structure enumerated in the preceding 

 portion of this paper extend to comparatively short distances from given 

 centres of chemical action : those I am about to notice (especially the trans- 

 verse cleavage of various slate rocks,) are of a very dififerent character. As 

 the finest examples of slaty cleavage are derived from the great Cumbrian 

 cluster of mountains, and from the chains of North Wales, it may be well, in 

 the first place, briefly to compare the physical structure of the two regions. 



The zone of Cumbrian green slate alternates with an indefinite number of 

 tabular or stratified masses of felspathic and ])orphyritic rocks. The slates are 

 crystalline, and have been so firmly packed in among the alternating por- 

 phyries, as to undergo very few contortions or undulations during the period 

 of their elevation. Moreover, they contain no organic remains: such remains 

 being, perhaps, obliterated; or, more probably, organic beings not having 

 propagated in an ocean exposed to continual incursions of felspathic rocks. 

 All the masses alternating with the porphyritic system exhibit in greater or 

 less perfection a cleavage, which is in no instance parallel to the true beds. 

 These facts were stated in a former paper*; and I expressed my belief that 

 the felspathic tabular masses were of Plutonic origin, and that even the great 

 alternating beds of slate (especially the more crystalline and chloritic varieties), 

 might, in part, have derived their materials from Plutonic sediment. I also 



■ * See Proceedings, vol. i, p. 400. 

 VOL. III. — SECOND SERIES. 3 P 



