The nature of the difference between the flow and non-flow arrangement of the felspar-lathes is well brought out in some dykes of basalt and augite-andesite that I examined in this island and also in the Valle del Bove on the Etna slopes. In the outer vitreous portion the felspar-lathes, which are fairly well represented, are all about the same length and are more or less parallel with the sides of the dyke. In the central more crystalline portion two sets of 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 consolidation 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 illustrated in a dyke must be postulated for all intrusive masses; but I have not yet found it possible to make much use of it. Much ground will have first to be cleared before it can be safely employed, since it is apparent, for instance, that there are often all gradations in a slide between a lathe and a phenocryst, and that the term “phenocryst” is applied to crystals having very different histories.
[Illustration: Diagram illustrating the two sets of felspar-lathes in a dyke: _A_, long and parallel, dispersed through the mass. _B_, short and non-parallel and found 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.
(_b_) This character is as a general rule associated with the plexus or non-fluidal arrangement of the felspar lathes.