64 PEOCEEDINGS OP TBCE GEOLOGICAL SOCIETT. 



disengaged, and thus crystallization, and previous fusion (if the rock 

 has not always been molten) have taken place in the presence of 

 water. Thus the controversy is either illogical, if restricted to 

 granite, or merely a verbal one, if extended to other plutonic rocks. 

 But some will assert that granite may justifiably be considered 

 alone, because many cases exist in which a perfect passage can be 

 traced from normal granite into normal gneiss. As regards this 

 assertion, I repeat the remark already made, that these cases are 

 less numerous than is supposed. I have examined a good many 

 asserted cases of transitions from igneous to sedimentary rocks, and 

 found that, as a rule, the distinction between the two rocks was well 

 marked, and that the generalization was the result either of precon- 

 ceived theory or of too hasty observation ; further, that precisely 

 similar difficulties exist with syenite and diorite, though the cases 

 are less numerous, as the rocks seem not quite so common ; lastly, 

 that those cases which appear to favour the theory occur only in 

 the case of rocks which there is good reason to believe are among 

 the most ancient yet discovered. These, as I have said, I lay aside 

 for the present. Eut in the case of the Palaeozoic and later rocks 

 it is only rarely, and under circumstances suggesting subsequent 

 obliteration of the characteristic structure, that I see a difficulty in 

 distinguishing between rocks of igneous and of non-igneous origin. 



I shall not then dwell further on the question of the origin of an 

 igneous rock, but I shall use the term to signify one that has solidi- 

 fied from a state of fusion, due to the existence of an elevated 

 temperature, whether we might call this dry fusion or not. 

 Further, in my classification, I shall draw no line of demarcation 

 between igneous rocks of Tertiary and Post-Tertiary age and those of 

 earlier date ; the reasons for this I defer, and think myself justified 

 in so doing, because it is one which logically would not arise in any 

 independent study of petrology, but would be imported into it as 

 the result of conclusions arrived at from other considerations not 

 entering into my classification. 



Theoretically, and probably also in practice, there are at least 

 two well-marked physical conditions in which any rock of a definite 

 chemical composition may occur, the hyaline and the holocrystalline. 

 Is there an intermediate condition wherein the rock, though no 

 longer vitreous, though exhibiting some differentiation of consti- 

 tuents, has not attained such complete individualization as to justify 

 the use of the term holocrystalline ? I do not allude to the interpo- 

 sition in the glass of a great number of microliths of individualized 

 minerals, although this may offer obvious practical difficulties, nor do 

 I refer to the size of the crystals making up the holocrystalline rock. 

 I should deem the latter epithet properly applied, whether the cha- 

 racter was readily observed with the unaided eye, or could only be 

 seen under the microscope; but I speak of cases in which the 

 material appears to have lost the usual property of a colloid, to have 

 acquired, around innumerable centres, polarities in one or more 

 directions, without its being possible to distinguish with precision 

 what are the minerals into which it has segregated ; where, in fact, 



