ANNIVEESAET ADDEESS OE THE PEESIDENT. 57 



certain stage of knowledge has been reached, even hasty traverses 

 of a district, which are much to be deprecated at an earlier period, 

 become of the highest value. For instance, there are portions of the 

 Alps in which I should now learn more by walking along the 

 chain in a zigzag course over four passes, than I should by spend- 

 ing the same time in the minute scrutiny of any one of them ; 

 because in the latter case I should probably only obtain additional 

 instances of things already known, while the former would enable 

 me to draw some general conclusions. 



In this address I shall not attempt to lay before you a tithe of the 

 evidence on which some of the statements which I may make are 

 founded, because to enter upon minute details like those of micro- 

 scopic work would expand it beyond all reasonable limits ; and 

 because, as I have said, they would fail to appeal to many of my 

 audience. It will be my endeavour to give, as briefly as possible, 

 a sketch of the nature of the answers which my investigations have 

 returned to certain questions which, though they did not arise quite 

 simultaneously, were ultimately prominent in my mind. These 

 questions were the following : — 



(1) What connexion is there (if any) between stratification, 

 cleavage, and foliation ? 



(2) What are the relations between the rocks commonly con- 

 sidered of igneous origin and those presumably of sedimentary ? Do 

 members of the one class ever pass into the other ? Or, in other 

 words, are we right in regarding the former as the result of meta- 

 morphic action on the latter, carried to an extreme ? 



(3) Are the rocks commonly called metamorphic, restricted to any 

 period or periods in geological history ? 



(4) What is the genetic history of the rocks ^ commonly denomi- 

 nated metamorphic ? 



But before attempting to contribute, however inadequately, to 

 the replies to these questions, I must venture a few remarks upon 

 the nomenclature of this branch of petrology. Last year I com- 

 mented upon our difficulties and deficiencies in this respect as re- 

 gards igneous rocks ; but the confusion which there prevails is small 

 in comparison with that in the present department. 



Eor instance, two words of primary importance, metamorphism 

 and schist, are as yet far from being used in a definite sense. One 

 geologist will speak of the rocks of a particular district as " meta- 

 morphic," or exhibiting " regional metamorphism," when they are 

 only silky slates and indurated grits, which may perhaps be termed 

 quartzites : rocks, that is to say, in which subsequent chemical 

 change has utterly failed to obliterate the indications of an original 

 clastic condition. Another will mean by the phrase that the rocks 

 are gneisses and crystalline schists (micaceous, hornblendic, &c.) ; 

 that is, rocks in which the original constituents have been practically 

 effaced by subsequent chemical changes. Again, in the mouth of 

 one geologist, a " schist " will mean any rock that has a rough 

 fissility, — probably not a shale, but either an imperfectly cleaved 

 ^late- or a foliated rock ; while another restricts the term to the 

 foliated rocks, in which, as a rule, no obvious traces of the original 



