SCHISTS OF THE LIZARD DISTRICT. 1 9 



is an original constituent ; crystallization in situ, especially in the 

 case of the hornblende and mica, has taken place to a large extent. 

 In the more minutely crystalline schist, the original structure is 

 very probably wholly obliterated. Still, these larger felspar grains, 

 for instance, may have as their nucleus felspar grains which were 

 original constituents, and may have survived the dissolution of the 

 finer sedimentary materials in which they were imbedded. Then 

 in the process of reconstitution, felspar (not perhaps always of the 

 same species) may have been added to felspar, quartz to quartz, 

 mica to mica, and hornblende to hornblende or altered augite. 

 Thus traces of the minuter structure of the original rock, even in a 

 highly metamorphosed series, may now and then remain. In those 

 beds where the chemical composition of the constituents facilitated 

 change, or where the materials were finely levigated, the agents of 

 metamorphism reduced the whole to a mere pulp (if the expression 

 be permissible) from which the present mineral constituents crys- 

 tallized, almost as they would do from the magma of an igneous 

 rock ; but in other cases only a portion of the material was reduced 

 to this condition, and those constituents which remained undigested 

 would form nuclei around which the other minerals would crystal- 

 lize, and would so continue to bear testimony to the original history 

 of the rock itself. Thus the explanation of those granitoid bands, 

 in some cases so curiously like granite veins, may be, that originally 

 they were a rather coarse quartz-felspar grit. As regards some of 

 the hornblende schists, one would suggest the possibility (as I believe 

 has been elsewhere done by the late Prof. Jukes) of their having 

 been basaltic tuffs, with which in chemical composition they would 

 agree fairly well*. 



I have ventured on this digression because these Cornish rocks 

 have presented structures which seem to me worthy of careful con- 

 sideration by all who are studying the phenomena of metamorphism 

 — a subject which has occupied my attention for some years past. 

 The observations are not entirely novel. Dr. Sorby drew attention 

 to somewhat similar structures in his very valuable and suggestive 

 paper on the original constitution and subsequent alteration of 

 mica-schist f . The agglutination of identical mineral matter has 

 been noticed in the case of quartz by that author, by Mr. J. A. 

 Phillips, and by myself, independently, not to mention others. In 

 the gneissic series traversed by the upper part of the St. Gothard 

 Pass, and in other districts, I have repeatedly noticed similar in- 

 stances, all tending to show that the minute structures, and in some 

 cases very probably the original constituents (at any rate as nuclei), 

 may be preserved in rocks which are metamorphic in the fullest 

 sense of the word. 



* See analysis byMr.Hudleston in the Appendix to my former paper (vol. xxxiii. 

 p. 924). While examining this hornblendic group I had always present to my 

 mind the possibility that the more massive varieties might be really metamorphic 

 igneous rocks; but I obtained no evidence in favour of this view, and am dis- 

 posed to regard the group as, at any rate in the main, of sedimentary origin. 



t Q. J. G. S. vol. xix. p. 401. See also many most suggestive and valuable 

 remarks in his Presidential address, vol. xxxvi. 



n 9. 



