as exhibited in the Devonian Limestones of Devonshire. 23 



Hfracombe No. 5. (Physical analysis.) 



Quartz sand 7'6 



Broken fragments of encrinites 1"6 



Peroxidized pyrites 1*3 



Patches of organic clay with inorganic impurities . 16' 7 



Crystallized calcareous spar, derived from organic clay 72*9 



100-0 



The above analyses are of very characteristic limestones, but 

 others occur whose composition is intermediate between them ; 

 and there is every gradation to pure organic clays, of which I 

 need not give any examples, for they have no decided peculiari- 

 ties except what I shall refer to presently. 



I now proceed to the changes of chemical composition. Many 

 of the limestones are more or less dolomitized, a portion of the 

 carbonate of lime having been removed and its place occupied 

 by carbonate of magnesia, so as to produce a mineral of a che- 

 mical composition distinctly diflferent to that of the original 

 deposit of organic sand or clay. In a similar manner, in some 

 cases the carbonates of the protoxides of iron or manganese have 

 been introduced so as to form brown-spar, which have often be- 

 come peroxidized by the action of the oxygen of the atmosphere, 

 after the rocks were elevated from their original positions, so 

 that they now exist as higher oxides, uncombined with carbonic 

 acid, in the form of granules in the solid calcareous crystals. 

 I shall not now enter into a consideration of the cause of these 

 chemical changes, but, when they have occurred, I see no reason 

 why the rocks should not be called metamorphic ; though, in de- 

 scribing them as such, it must not be thought that I confound 

 this kind of metamorphism with such as has taken place in the 

 vicinity of igneous rocks. Moreover, since in this change lime- 

 stones have in some cases been altered in such a manner as to 

 have plates and flakes of a different mineral composition or cha- 

 racter developed in them, I see no reason why such should not 

 be called foliated, and why this term should be restricted to the 

 production of mica and such other minerals as are met with in 

 rocks more commonly called metamorphic, in which chemical 

 changes have probably been induced by the agency of heat, 

 whereas in the others they were most likely produced by actions 

 taking place at a temperature not much above the natural. 



In some cases the difference between calcareous spar and do- 

 lomite is sufficiently well marked, but in others they cannot be 

 distinguished by the microscope, and it is necessary to resort to 

 chemical analysis. Those that I give are intended to illustrate 

 physical facts, and not the mere chemical composition, and I 



