198 Prof. T. G. Bonney — Foliation and MetamorpJdsm. 



I have consulted memoirs and textbooks only so far as to obtain 

 a statement of the problems and to ascertain what places were likely 

 to throw light upon them, and on this account ask my readers, if 

 they think my conclusions wrong, before rejecting them 

 contemptuously because they are unorthodox, to "go and see" — to 

 examine nature for themselves, to collect their own specimens, and to 

 study them with the microscope. 



Foliated Rocks. 



Foliation denotes a structure due to a more or less parallel ordering 

 of certain of the mineral constituents in a rock which is not a direct 

 consequence of its stratification. Such rocks are commonly called 

 metamorphic, but that structure, as we shall see, is more often 

 an original one than was once supposed. In a certain sense all rocks, 

 except recent volcanic ejections and such sedimentariesas wind-blown 

 sands and dusts, or diatomaceous earths, coral-reef limestone and 

 even pure chalk, are metamorphic, because they have undergone 

 some amount of mineral change since the time when they were first 

 deposited. In a sandstone, for instance, the fragments are cemented 

 by a little secondary quartz or iron-oxide ; in a limestone those of 

 calcareous organisms are united by calcite ; in an igneous rock the 

 less stable minerals have undergone some change, but we do not 

 consider this as amounting to metamorphism. To justify the use 

 of that term, the bulk of the constituents, all in fact which 

 are small in size, should have lost the aspect which was once 

 characteristic ; for instance, in a limestone the calcareous organisms 

 should be no longer recognizable as such, and any constituent mud 

 should have been converted into a mica or some other authigenous 

 mineral. This may, however, happen, as we shall presently see, 

 without resulting in a definite foliation. But in certain igneous 

 rocks such a structure may be, not a superinduced, but an 

 original one. 



Foliation, then, is produced by a parallel ordering of the 

 authigenous platy or acicular constituents in a rock, and may or may 

 not be associated with mineral banding. 1 In the simpler case, when 

 the constituents are not thus grouped, we find that the foliation is 

 the result of movements in the mass, before it has become completely 

 solid, though after most of its component minerals had segregated. 

 Banded foliation, however, proves to be due, at any rate in many 

 cases, to the movement of two associated magmas, different in 

 chemical, composition, both of which are in a somewhat plastic 

 condition, though one may be more nearly liquid than the other. 

 The two are forced along, sometimes without mixing, as two 

 differently coloured glasses may be in the hands of a glassblower. 

 Many varieties of rock, from acid to basic, from vitreous to 

 holocrystalline, afford examples of this structure. 



I have studied its various stages in several places, of which the 

 following may serve as examples, [a) On the southern half of 

 Kennack Cove (Lizai'd) we can see veins of a grey granite breaking 

 up a dark slightly foliated biotite-diorite, which it partly melts 



1 See Manual of Geology, J. B. Jukes, 1872 (ed. A. Geikie), p. 142. 



