Prof. T. G. Bonney — Foliation and Metamorphism. 249 



rock subsequently metamorphosed. Originally a peridotite, 1 and 

 frequently occurring in large masses, as at the Lizard and in the 

 Alps, it has been converted by gradual change into a mass of flaky 

 serpentine, as was described in my first paper on the Lizard, to 

 which I have already referred, 2 and this rock also, by further 

 action of water, may be altered with a talc-schist (46). An 

 exceptional kind of chlorite-schist which occurs, apparently 

 intrusive, in taloose-serpentine on the Gorner Grat and in Anglesey, 

 has once been, as already mentioned, a diabase, and dykes of the 

 latter can be seen at the Lizard occasionally to pass into varieties 

 of "potstone", while the so-called white-trap is another modifica- 

 tion of a basaltic rock too well known to need more than mention. 



The metamorphic rocks still present difficulties — points in their 

 history which require further elucidation — but during the last 

 half-century, as I know from my own experience, and as any 

 one can ascertain by consulting the books and papers, which at 

 the beginning of that period were regarded as authorities, the 

 mists which then obscured knowledge have been largely dispersed, 

 and many misleading and erroneous notions have been banished. 



The passage of sedimentary into igneous rocks, once so confidently 

 asserted, has proved to be no better than a figment of the imagina- 

 tion. It is possible, of course, that a sedimentary rock may be 

 melted down, especially if small fragments of it are caught up in 

 large masses of molten material, but even these appear, as a rule, 

 to be so refractory that little evidence can be found in its favour. 3 

 The gneiss and schists, the rocks commonly called metamorphic, 

 prove, as a rule, to be more ancient than any strata to which a date 

 can be assigned, and belong, apparently, to an era in the earth's history 

 anterior to the appearance of life, when the temperature of its crust 

 rose more rapidly in a downward direction than at the present day. 

 This statement, fifty years ago, would have been scouted as heretical 

 by most geologists ; I think, however, that the bulk of those who 

 have studied petrology, not onlj' in the field, but also with the micro- 

 scope, would now consider it to be the more probable hypothesis. 



Bibliographical References. 

 I have omitted my own name where, as in the majority of cases, the papers 

 are written by myself. Q.J. denotes Quarterly Journal of the Geological 

 Society, G.M. Geological Magazine, M.M. Mineralogical Magazine. 



1. Q.J. xlvii, p. 476; Hi, pp. 22, 27-30; "Crystalline Schists of the 



Lizard," pp. 21-33. 



2. Q.J. xxxiii, p. 888 ; xxxix, pp. 11, 16. 



1 I believe it is generally a deep-seated rock which occurs more often in 

 bosses than in dykes, and never, so far as I have been able to ascertain, as 

 a flow. 



2 Perhaps the minutely flaky form of serpentine named antigorite may be 

 due to subsequent pressure during or subsequent to the action of water (47). 



3 The dark patches in igneous rocks have been discussed in an excellent 

 paper by Mr. J. A. Phillips (Q.J. xxxvi, pp. 1-22), but I regard them, more 

 often than he has done, as remnants of fragments of much older rocks, or pieces 

 of a more basic igneous rock that have been broken off and carried along by 

 the moving magma. 



