238 A NATURALIST IN THE PACIFIC chap. 



lathes can be distinguished, one (A) corresponding in the average 

 length and in the flow-arrangement with the lathes of the borders, 

 the other (B) being about half the length and forming a plexus 

 between the larger parallel lathes. Those of the A set, which are 

 those that usually catch the eye in a section, are contemporaneous 

 in their origin with those in the margins of the dyke ; whilst those 

 of the B set have been subsequently formed. In the preliminary 

 " stiffening " of the first stage of consolidation, the whole mass of 

 the dyke would be affected. To this stage the lathes of the A set 

 belong ; whilst to the later stage of con- 

 solidation which would proceed much more 

 slowly in the interior than at the margins of 

 the dyke, the lathes of the B set are to be 

 referred. This distinction so plainly illus- 

 trated in a dyke must be postulated for all 

 intrusive masses ; but I have not yet found it 

 Centre possible to make much use of it. Much ground 



Diagram illustrating the will have first to be cleared before it can be 

 kthesTn a^dykeT^' safely employed, since it is apparent, for in- 

 A, long and parallel, Stance, that there are often all gradations in 

 dispersed through the ^ gij^jg between a lathe and a phenocryst, 

 £, short and non- and that the term "phenocryst" is applied 

 parallel and found ^q crystals having very different histories. 



only in the centre. / ^ / ,..,,. 



With regard to the ophitic habit of some 

 of the basaltic rocks the following conclusions may be drawn : 



(a) Typical ophitic " plates " are not very common in the slides. 

 More frequently the habit of the pyroxene is semi-ophitic. 



(d) This character is as a general rule associated with the 

 plexus or non-fluidal arrangement of the felspar lathes. 



(c) The felspar-lathes are nearly always large, frequently 

 averaging more than '2 mm. in length. This coarse doleritic 

 groundmass is almost diagnostic of an ophitic rock. 



It is not always possible to allow for the influence of locality in 

 drawing up such a classification as this, since it is well known that 

 in each volcanic region the rocks have a particular facies recognis- 

 able in hand-specimens as well as in the slide, though it is not easy 

 to express such a distinction in a definition. Perhaps this is 

 represented in " adaptation " as we find it in the organic world, and 

 the question arises as to the value of such characters for critical 

 purposes. Regional variation plays such an important part that it 

 cannot be ignored in rock-classification. 



