THE SlENTNG DISTKICTS OF CORNWALL. ' 343 



deposits of ore parallel with the line of the dip of the adjoining 

 country, would, however, lead to the conclusion that lateral infiltra- 

 tions must have materially influenced the results. 



8. Contact-deposits and stockwerks have been formed by analo- 

 gous chemical action, set up, in the first case, in fissures resulting 

 from the junction of dissimilar rocks, and, in the second, in frac- 

 tures produced during the upheaval of partially consolidated erup- 

 tive masses. The alteration experienced by stratified deposits in the 

 vicinity of eruptive rocks is probably often due to somewhat similar 

 percolations. 



9. It does not appear improbable that quartz may sometimes 

 retain a certain amount of plasticity after it has assumed a crystal- 

 line form, and that it subsequently hardens. 



EXPLANATION OF PLATE XVI. 



Magnified 15 diameters. 



Fig. 1. An imperfect crystal of quartz from the elvan of Trerice quarry, Crowan. 

 No fewer than six separate intrusions of felspathic base have taken place 

 into its substance ; in one instance the enclosed base has become nearly 

 detached in the form of a pear-shaped bleb, while in another the 

 almost totally separated fragment has assumed a quasi-crystalline 

 form. 



2. A crystal, from the same locality, in which several distinct intrusions 



appear to have coalesced, while the openings by which they were ad- 

 mitted have again closed. 



3. Specimen from the same elvan ; it contains a considerable number of 



spherical particles of base, upon which the periphery of the crystal 

 has closed after their admission. 



4. Quartz crystal, from the Trerice elvan, in which all the detached por- 



tions of base have been compressed into quasi-crystalline forms. 



5. A patch of crystalline quartz in capel from Botallack ; this presents the 



appearance of having been broken while still in a somewhat plastic state, 

 and the fissure afterwards filled with schorl, crystals of which have 

 penetrated into the substance of the quartz. 



Discussion. 



Prof. Ramsay concurred with the author in his opinion as to the 

 origin of lodes. It has sometimes been maintained that all lodes 

 are of igneous origin, whereas it seemed to him that the materials 

 of all the lodes he knew in various mining districts have been de- 

 posited from solution. 



Prof. Hull thought that the author was on the right track, as he 

 was employing both microscopical and chemical tests in investigating 

 the characters of the rocks. The mapping of contemporaneous vol- 

 canic rocks will be hereafter of the greatest interest. He expressed 

 a hope that the term " greenstone " will soon be extinguished, as it 

 is not a name with a definite meaning. 



Mr. F. Rutley made some remarks on the various rocks to which 

 the name " elvans " had been applied, granites, felstones, and quartz- 

 porphyries having been indiscriminately termed elvans. The name 



