414 EEPOET — 1888. 



more complete than others. This must have been produced under the 

 action of a directed force, which caused no motion in the mass as a whole, 

 and may therefore be called statical. 



14. The effect of pressure on a crystalline rock is to produce spectral 

 polarisation, and a further amount to produce microspectral polarisation. 



15. In the case of felspars, a structure closely resembling that of 

 microcline is often found in rocks which have been subjected to pressure, 

 and may be due to that pressure. 



16. Pressure accompanied by shearing produces mylonitic lines, which 

 may influence subsequent crystallisation, but which are quite distinct 

 from ordinary orientation, the force brought to bear resulting in motion 

 of the mass, and being therefore dynamical. 



17. The pressure under which rocks have become foliated is greater 

 than that necessary to produce microspectral polarisation. 



18. Some of these ancient rocks have been produced by the agency 

 of calcareous and siliceous springs. 



19. The chemical composition of the i-ocks considered igneous is 

 different from that of the general sedimentary rocks, and often includes 

 special minerals. 



20. The elements of the holocrystalline igneous rocks of acid type are 

 of a different order of magnitude from those of the sedimentary rocks. 



21. Such acid igneous rocks are of various mineral constitution and 

 have undoubted felsites intimately associated with them, 



22. Some of the most peculiar of the crystalline rocks are altered 

 volcanic products without orientation, 



23. The basic igneous rocks have a great tendency to foliation, and 

 may become as foliated as any schist. 



24. Some of the hornblende schists may be altered dolerites, without 

 showing any signs of subsequent pressure or shearing. 



25. The serpentines are derived from more than one variety of 

 original rock, including saxonite and Iherzolite. 



26. Igneous rocks, including gabbros, have their mylonitic represen- 

 tatives, whereby they may become schistose, and the production of 

 schistosity may be accompanied by chemical changes. 



27. The difference in character between these and the regularly foli- 

 ated rocks indicates a different form of pressure in the two cases, the 

 latter being practically unaccompanied by shearing ; and standing, as in 

 the case of the sedimentary rocks, in the relation of a statical to a 

 dynamical force. 



28. Almost all varieties of rock have their cataclastic representatives, 

 unaccompanied by any radical chemical change. 



29. Many rocks have very long histories, the events of which have 

 left their traces in the structure, and all of which may be subsequent to 

 their foliation, 



30. The dykes and isolated masses have peculiar characters — macro- 

 felsitic, granophyric, variolitic, &c., distinguishing them on the one hand 

 from the more ancient crystalline rocks, and on the other from the newer 

 dykes of the district, 



31. An irregular arrangement of minute elongated crystals — in other 

 words, a spicular structure — is favourable for the production of spheroidal 

 structure, as indicated by the mode of weathering. 



32. Finally, the whole series of rocks presents many original forms 

 which are still recognisable, and the metamorphosis they have undergone, 



