46 



PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



On the Structure akb Origii!^ op Non-calcareous 

 Stratified Eocks. 



TABLE OF CONTENTS. 



Introduction page 46 



Origin of material '47 



Quartz -sand 47 



Sand derived from other minerals. 49 

 Mechanical wearing of material ... 50 

 Chemical decomposition of mate- 

 rials 50 



Minute calcareous granules 51 



Pseudomorphs after felspars 51 



Identification of some minerals ... 52 



Mica in deposited rocks 53 



Augite, Hornblende, and Olivine in 



deposited rocks 53 



Volcanic glass and ashes in depo- 

 sited rocks 54 



Microscopical examination of 



Sands and Clays 55 



Identification of the Minerals in 



Sand 55 



Identification of fragments of rocks 



in Sand 56 



Plint-sand 56 



General characters of Sands 57 



External form of the grains 57 



Variously worn Sands 58 



Fractured grains 61 



Corroded Sands 61 



Sand with quartz chemically de- 



posited on the surface of the 



grains page 62 



Internal structure of the grains ... 64 



Mud and Clay 66 



Lamination of Shales, &c 67 



Slate rocks in general 68 



Slate rocks of North Wales 70 



Development of Slaty Cleavage ... 72 

 Slates of the English Lake-district 74 



Cleavage in the Green Slates 76 



Slates in different districts 76 



General changes after deposition. , , 77 



Metamorphic rocks 81 



Artificial Metaraorphism 81 



Local-contact Metamorphism 81 



Development of Mica-schist 82 



Fine-grained Schists 84 



Foliation in slightly altered Slates 84 



Structure of Schists 85 



Presence of the original Sand and 



Mud in Schists 85 



Original source of the material of 



some Schists 87 



Stratification-foliation 88 



Cleavage-foliation 89 



Schists not mechanically deposited 



as such 91 



Conclusion 92 



Introduction. 



In my address at our last Anniversary Meeting, 1 gave a general 

 outline of what appeared to me to be the most important facts 

 connected with the structure and origin of limestone. Having 

 treated the question mainly from a new point of view, I did little 

 else than describe my own observations. On the present occasion 

 I propose to treat mainly on the structure and origin of non- 

 calcareous stratified rocks, and to consider only those questions 

 connected with igneous rocks which bear very directly on the 

 subject before me. Some branches of this inquiry have already 

 attracted much attention ; but it would extend this address too 

 much to discuss what has been writen on one detached portion or 

 another. It will, I think, be better to give a summary of what 

 appear to me to be the more important facts, as viewed from a 

 general, and yet somewhat special, point of view. In doing this I 

 must necessarily allude to many facts and conclusions which can 

 have no claim to novelty, except in having been arrived at by more 

 or less new methods, or in being discussed in a new counexion, or as 

 bearing on more general questions. 



