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



stratification of the neighbouring country. The structure of the rock has 

 been so modified that the traces of its original deposition are quite obhterated; 

 and this remark does not apply merely to single quarries, but sometimes to 

 whole mountains. In many slate quarries, and even in hand-specimens of 

 slate, we can, however, discover a number of parallel stripes, sometimes of a 

 lighter, and sometimes of a darker colour than the general mass; and in 

 rocks of the age I am considering, these stripes are universally parallel to the 

 true bedding of the rocks. The proof of this is established by the fact, that 

 the assumption leads to consistent results ; and that these stripes are always 

 parallel to true beds, whenever such beds can be discovered, whether by orga- 

 nic remains, by the alternations of dissimilar deposits, or by any other ordinary 

 means. I have seen thousands of examples of the truth of the rule, and not 

 one exception to it, among rocks of the age I am considering. Sometimes 

 all these means fail, and we may ramble for miles among mountains of slate 

 without seeing a single trace of their original stratification. 



In examining a formation of greywacke, we may find thick well-defined 

 beds, passing into thin flaggy beds ; and these, again, passing into masses, 

 subdivided into very thin laminas. These thin lamincc often resemble the 

 coarser varieties of slate, and are, indeed, sometimes used for the same pur- 

 poses. There may, therefore, be cases where, as far as mineral structure is 

 concerned, slatestones of cleavage, and flagstones, which are thin beds, can- 

 not be separated from each other. These cases are, however, very rare 

 exceptions. K flagstone is generally distinguished from a true slate, by slight 

 deviations in its plane ; occasionally by what is called the ripple mark ; by a 

 dull granular surface; by scattered flakes of mica, entirely unlike the con- 

 tinuous chloritic flakes of a true cleavage; and sometimes by organic remains 

 studded on its surface. By such indications as these, and by the undefinable 

 power acquired by habit, a Welsh quarry-man, accustomed to work in the 

 upper division of the schistose groups, seldom fails to separate the laminas of 

 deposition from true slates ; and in the same quarry he will point out the 

 distinction between the planes of stratification and the planes of cleavage*. 



1 think it obvious that the contortions of slate rocks are phasnomena quite 

 distinct from cleavage, and that the curves presented by such formations are 

 the true lines of disturbed strata. In many cases a cleavage seems to have 

 been the last change superinduced on rocks before they became entirely 



In many parts of Cumberland and Caernarvonshire, where the porphyries interfere with the 

 slates, and where the slaty structure is so completely developed as almost to obliterate the traces 

 of stratification, these distinctions are entirely unknown among quarry-men. 



