AlfNIVDKSAKT ADDEESS OF THE PEESIDENT. 65 



Meuse are typical examples of the rock of which I speak. But in 

 the case of rocks originally crystalline, ^vhether igneous or so-called 

 metamorphic, this superinduced formation of minerals appears to 

 take place more readily, because the original constituents of the 

 rock are favourable to it ; certain hydrous micas forming readily from 

 felspar, hornblende from augite, serpentine from olivine, chloritic 

 minerals from black micas, or in some cases from augite or horn- 

 blende, &c. In consequence of this a very conspicuous cleavage- 

 foliation may be produced in a crystalline rock which may or may not 

 have exhibited stratification-foliation. In this case, however, the 

 individual minerals are, as a rule, rather minute, e. g. the cleavage- 

 surface produced in a granite or gneiss becomes coated with a film 

 of minute flakes of silvery or greyish micas, such as sericite, 

 damourite, paragonite ; the exact species being often hard to de- 

 termine, and of course not always the same. The rock is more or 

 less fissile, often markedly so, parallel with this surface of foliation, 

 and the new mineraV structure is often far the most conspicuous to 

 the unaided eye. Yet after all it is frequently very superficial. We 

 may take a slab of rock which appears to be a typical grey mica- 

 schist, but examination of a transverse section proves it to be a true 

 gneissic or granitic rock, cleaved first and then " varnished " by this 

 new mineral deposit. It is this secondary foliation which I propose 

 to call " cleavage-foliation," and to refer to its surface, often one of the 

 most conspicuous in a mountain-district, as the " sheen-surface," to 

 distinguish it from an ordinary cleavage-surface on which there has 

 been no notable amount of mineral rearrangement. The efi'ect of this 

 foliation will, of course, vary with the amount of pressure, the con- 

 stituents of the rock, and a number of other circumstances. It may 

 impress a distinct foliated structure on a rock hitherto not foliated, or 

 it may coexist with an older foliation ; it may cut through the latter 

 at various angles, or it may, if I be permitted the phrase, intensify it ; 

 in a word, it will exhibit the same relation to the original structure 

 of crystalline rocks that cleavage does to stratification (see p. 97). 



Moreover we remark among the crystalline rocks another result 

 of pressure analogous to that which it produces in sedimentary 

 rocks. In the latter, divisional planes are often noticed, parallel 

 with the cleavage-planes, but of less cohesion, so that the mass 

 assumes a deceptive appearance of bedding. The former also often 

 exhibit what seem to be indubitable indications of bedding, parallel 

 with the cleavage-foliation ; but these, on examination, will prove 

 to be quite deceptive. It would be convenient to have a term for this 

 structure : as " false-bedding " is already appropriated, I fear I must 

 take refuge in Greek, and would suggest pseudostroraatism. Moreover, 

 as crystalline masses, whether from slight difference in coarseness 

 or constitution, or from the existence of joints, are of unequal 

 strength, we find that not seldom the results of pressure arc more 

 marked in some parts of the mass than in others ; so that beds 

 of mica-schist, apparently interstratified, are locally developed by 

 crushing in a granitoid gneiss ; zones also of fine-grained rock with a 

 " slabby " bedding occur, which, at first sight, would be unhesi- 



