[ 445 ] 



LVI. An Inquiry into the Nature of the Structure of Rocks. 

 By Henry S. Boase, M.D., fyc. 



[Continued from p. 383.] 



T> UT let us endeavour to advance a step further in the prac- 

 **■ tical application of the new system to rocks in situ, to the 

 roofing-slate, for example, in one of the most extensive slate 

 quarries of Wales, where the characters of the rock are all 

 fresh and well defined, on account of recent and extensive ex- 

 cavations. Is this a slate ; that is, has it a perfect cleavage ? 

 According to the old phraseology this would be deemed a 

 very simple question ; but now it can only be answered by 

 another interrogatory, What is the direction of the strata ? 

 Prof. Sedgwick gives the following answer : " Of all places a 

 slate-quarry is often the very worst for determining the strati- 

 fication of the neighbouring country." The reason of this we 

 are told, in another place, is because " the structure of the 

 rock has been so modified that traces of its original deposi- 

 tion are quite obliterated; and this remark does not apply 

 merely to single quarries, but sometimes to whole mountains." 

 Without stopping to inquire by what indications it was ascer- 

 tained that the marks of deposition once existed, or by 

 what possible modification of structure they could be obli- 

 terated, we must proceed with the question under considera- 

 tion, viz. whether the Welsh roofing-slate has a true cleav- 

 age? A little further on the Professor informs us that 

 * sometimes all these means (the ordinary means of discover- 

 ing strata) fail, and we may ramble for miles among mountains 

 of slate without seeing a single trace of their original stratifi- 

 cation." 



It therefore follows, that the apparently self-evident ques- 

 tion whether a rock be fissile or has a slaty structure, must 

 be often left undetermined, since a few miles' ramble will 

 frequently bring us not only on rocks having different bear- 

 ings, but even on entirely new formations. And the difficulty 

 will be greatly enhanced if the geologist should unexpectedly 

 alight on a good slate-quarry situated in an igneous formation, 

 such as the Roche Tiuliere in the Mont Dove; for the terms 

 slate, flagstone, and lamina? of the new vocabulary would be 

 inapplicable, in as much as the laminations or leaves can, in 

 their position, have no reference to that of strata, in an un- 

 stratified rock. 



It is not stated in the definition of the term cleavage, but 

 it may be gathered from the details (at pp. 471 and 473 Trans. 

 Geol. Soc. Second Series, vol. iii.), that the transverse cleavage 



