ORIG-inST^.Jl. AETICLES. 



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I. — Foliation and Metamokphism in Rocks. 



By Professor T. G. Bonney, Sc.D., LL.D.-, F.K.S. 



Introductory. 



rilHE application of the microscope to petrology made it possible to 

 I investigate effectively the history of the foliated rocks and of 

 metamorphism. This was done, as it happens, only a very few years 

 before I began any special study of such rocks, so that the account 

 of my own work almost corresponds with that of the general 

 progress in a knowledge of them. I was led, after some preliminary 

 efforts, into investigating two distinct problems, each of which, as 

 I soon discovered, presented rather exceptional difficulties. These 

 were the pre-Carboniferous rocks of Charnwood Forest and the 

 serpentines of the Lizard. The problem involved in the one was how 

 far some of them were or had been igneous in origin, lavas, tuffs, 

 and agglomerates, or were stratified rocks, which by pressure and 

 mineral changes had been so altered as to be indistinguishable from 

 some of the former. The other introduced the question of the origin 

 of serpentine, about which in 1873 the utmost uncertainty existed. 1 

 This, from its associations, led on to investigating the nature and 

 origin of gneisses and schists, so that the history of my own studies 

 during the last forty-five years happens to illustrate some important 

 aspects of the progress made during that period. So I have thought 

 that even now, notwithstanding the multiplication of textbooks 

 and special memoirs (nay, perhaps because of it), some younger 

 students may find it both interesting and useful to read the 

 •conclusions which I have been led to adopt, and to have their 

 attention directed to investigations which were, to a large extent, 

 independent and unbiassed. 



As my first paper depending on microscopic work was published 

 •early in 1877, anyone who consults those which have since appeared 

 must not be surprised to find that my conclusions have been reached 

 by degrees, and that some have had to be retracted. The former 

 should be forgiven, because one frequently resembled a man feeling 

 his way through a dense forest and the latter was often due to 

 misleading information. More than once I have found that the 

 old saying "put not your trust in princes" is true even in science. 

 Of both these defects my work in Charnwood and at the Lizard 

 affords instances. In the latter it was long before the real history, 



1 The following remarks appeared in Bocks, Classified and Arranged 

 {B. von Cotta ; translated by P. H. Lawrence and published in 1866 with 

 author's preface of same date). After stating that serpentine is "probably the 

 product of the metamorphosis of some other rock", he continues (p. 317) : 

 "In some places . . . its transmutation from other rocks is very evident, as, 

 for instance, from gabbro at Siebenlehn, near Freiberg ; from dykes of granite 

 traversing serpentine rocks near Bohrigen and Waldheim in Saxony, where the 

 main serpentine itself is not improbably a transmuted granulite ; from chlorite- 

 schist at Zell in the Fichtelgebirge, where the change does not appear to be 

 yet complete ; and from gneiss (probably) or an eclogite rock in the gneiss at 

 Zoblitz in the Erzgebirge." 



