64 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



older crystalline gneisses, we find a general orientation of the mineral 

 constituents, and a certain striping or banding of the rock, produced 

 by an excess of one or more of these over a zone ranging from a 

 fraction of an inch to at least several feet, which suggests original 

 differences of constituents due to bedding; this structure is sometimes 

 seen to be parallel to highly quartzose, micaceous, or calcareous layers, 

 which are most difficult to explain on any other theory than that of 

 stratification *. In such cases the rock usually is not markedly 

 fissile in the direction of these planes of mineral differences. I 

 shall call this kind of foliation, as it has been already named, 

 stratification-foliation, as being in some cases almost certainly, in 

 others probably, associated with stratification. There is, however, 

 another kind of foliation, often more conspicuous in the field, which 

 has in all cases been produced subsequently to the consolidation of 

 the rock, and as the result of pressure, generally lateral ; to this I 

 shall give the name of cleavage-foliation, a term which has been 

 already used by my predecessor, Dr. Sorby, though perhaps in a 

 rather different sense. I can define it best by describing the mode 

 in which it is produced. When fragmental rocks, such as sandstones 

 or shales, are subjected to great pressure, their constituents are to a 

 certain extent rearranged and flattened out until the mass assumes 

 the structure called " cleavage "f. In like way, as I shall presently 

 explain, crystalline rocks — whether granites, dolerites, felsites, rhy- 

 olites — in short igneous of any kind — or whether gneisses, schists, 

 &c., that is, such as usually are called metamorphic — when exposed 

 to like pressure, also assume a cleavage-structure. Modifications of 

 this structure I shall presently describe in more detail ; at present it 

 will suffice to say that, according to the nature of the rock, it is pre- 

 ceded in some cases by a great crushing of the crystalline constituents, 

 in others by fiexure terminating in rupture along planes approxi- 

 mately parallel. It results from this that we can have not only 

 cleaved granites, felsites, pitchstones, &c., but also cleaved gneisses, 

 schists, &c., i. e. that crystalline rocks may undergo a cleavage 

 equally with clastic rocks. Now, in the case of clastic rocks, we 

 occasionally see that in addition to the mechanical modification 

 resulting from pressure, there has been a slight amount of chemical 

 change, evidenced usually by the development of very minute 

 films of micaceous and chloritic minerals, especially along the sur- 

 faces of imperfect cohesion. The presence of these filmy minerals 

 gives to the slate a peculiar sheen, a sort of silky or satiny look, 

 which differences it from the ordinary slates, though it obviously is 

 much more nearly allied to them than to any of the true schists. 

 To these belong many of the phyllites, a term at jpresent vague, but 

 which I should like to see restricted to such rocks. The (presumed) 

 Cambrian slates of the Devillien and Eevinien series on the Upper 



^ The word is understood to be inclusive of chemical precipitates. 



t For a very able and interesting discussion of the precise nature of this 

 change, I refer to the paper on cleavage read before the British Association at 

 Aberdeen by Mr. H. Harker, F.Gr.S., and printed in extenso in the volume for 



1885. 



