E. Greenly — Diffusion of Granite into Schists. 209 



even the acid igneous rocks always presented difficulties, but basic 

 rocks and, I believe, even peridotites and serpentines were some- 

 times supposed to have originated in this way, as well as felsites, 

 which could not have consolidated under plutonio conditions. 



It is easy to be wise after the event, and for one period of human 

 thought to point the finger of scorn at the aberrations of its pre- 

 decessors ; and we must not forget that at that time hardly anything 

 was known of the microscopic structure of rocks, and very little 

 more of their chemical composition.^ When, therefore, in the light 

 of microscopic and chemical research the field evidence for many 

 of the alleged cases of transition broke down, it is not surprising 

 that the whole theory was cast aside, often with no little scorn, and 

 relegated to the limbo of exploded hypotheses. 



The remark has been made by Mr. Herbert Spencer that as there 

 is " a soul of goodness in things evil," so often is there, and that very 

 generally, " a soul of truth in things erroneous." And in this old 

 theory there was a soul of truth. 



It is noteworthy that most of the cases in which the evidence 

 so hopelessly broke down were those where the igneous rocks were 

 surrounded by tracts of ordinary sedimentary rocks that were only 

 locally, not regionally, metamorphosed. Eegionally metamorphosed 

 rocks had been, indeed, examined, and speculation aroused concerning 

 them ; but the time for systematic research into their phenomena had 

 not yet come. 



The past twenty years or so, however, have seen much energetic 

 and enthusiastic research into the crystalline schists, and really 

 scientific methods applied to their problems. Now, during that 

 period descriptions have been given, from, time to time, of a good 

 many cases where granitoid rocks which occur in districts of 

 regional metamorphism have been really observed to pass into the 

 surrounding gneissose rocks by perfectly gradual transitions. North 

 America, Scandinavia, Saxony, the Alps, more than one part of 

 the Scottish Highlands, Ireland, and even Anglesey, have furnished 

 examples." 



1 The time and labour demanded by analyses of silicates have always stood in the 

 way of a really thorough knowledge of the chemistry of rocks, and at the present 

 time hardly any work is so much needed in geology, if intelligently directed in con- 

 junction with microscopic and especially with field work. 

 ^ Lawson: Geol. Eainy Lake Region, 1888, pp. 118, 130, 137. 

 Van Hise : Pre-Camb. Rocks N. America, Corr. Papers, 1892, p. 488 ; and, quoting 



Jukes, Hitchcock, and others, p. 479. 

 Reusch: The Rommel and Karm Islands, 1888. 

 Lehmann : Enst. Altkryst. Sch., pp. 64, 67, etc. 

 Lory: Etudes Sch. Cryst. : Congres Inter. Geol., 1888. 

 Bonney: Pres. Address Geol. Soc, 1886, p. 51, etc. ; Two Traverses Cryst. Sch. 



Alps, Q.J.G.S., 1889, pp. 95, etc. 

 Barrow : An Intrusion Muscovite- Biotite Gneiss, etc. : Q.J.G.S., 1893, pp. 341, 



343, 353. 

 Home & Greenly : Fol. Granites and Cryst. Sch. East Sutherland : Q.J.G.S., 1896. 

 Cole : Metam. Rocks Tyrone and Donegal : Roy. Irish Ac, xxxi (1900). 

 Greenly: Sillim. Gneiss, Anglesey: Geol. Mag., 1896, p. 495. 

 Teall : Pres. Address Geol. Soc, 1902, p. Ixxiv. 



(These refei'ences are of course not exhaustive.) 



DECADE IV. — VOL. X. — NO. V. 14 



