180 Transactions of the Royal Microscopical Society. 



with one of these cracks (PI. CXXXIV.). Have we not here a struc- 

 ture analogous to the spheroidal structure sometimes seen in prisms 

 of hasalt ? The divisional planes in the basalt, which by their inter- 

 sections give rise to the prismatic forms, are assumed to result from 

 shrinkage or contraction consequent upon solidification. These less 

 regular little cracks in the perlites are unquestionably due to the 

 same cause. In a paper, read the other day, by the Kev. T, G. 

 Bonney, before the Geological Society, it was pointed out in a very 

 clear manner that the spheroidal structure in basalt results also 

 from contraction on cooling. Surely then if these spheroidal forms 

 in basalt are not intersected by the planes which divide the basaltic 

 prisms, but are enclosed by those planes, the prism-planes were the 

 first formed, while the spheroidal structure resulted from subsequent 

 cooling taking place along those planes. If this be a right inter- 

 pretation, I believe that we have here, in these perlites, a clearly 

 parallel case, and that the explanation of the spheroidal structure 

 in basalt is equally applicable to this, hitherto unexplained, structure 

 in perlite. 



I shall now point out those structures which seem most worthy of 

 remark in the section of a crystal of leucite which I have recently 

 examined. It is imbodded in a somewhat opaque lava which also 

 contains crystals of plagioclase, &c,, and was procured from one of 

 the lava streams of Vesuvius. Fig. 2 represents a few of many 

 minute enclosures, some of which are of very in-egular form ; 

 they have broad, dark margins, and seem to be devoid of bubbles, 

 so that I think they may possibly be glass lacunae. Fig. 3 is 

 approximately a straight cylinder, and either contains a plug of 

 granular matter or else is incrusted with it. Fig. 10 is a ball 

 of granular matter apparently perforated by a microlith or small 

 cylinder (similar figures are given in Zirkel's ' Mik, Beschaff. 

 d. Min. u. Gest.,' p. 75). Fig. 11 is a similar cylinder free from 

 granular matter. I speak of these bodies as cylinders, because 

 I am uncertain whether they are hollow or solid, but I think that 

 they are solid. Now the question arises whether Fig. 3 is simply 

 a version of Fig. 11, but containing a plug of granular ma- 

 terial, and if so, is Fig. 10 a similar cylinder to Fig. 3, but with 

 a bulb blown in its centre, just as we might heat a piece of 

 glass tubing and blow a bulb at that point? If merely an 

 incrustation around a rod, why is it that the rod is not equably 

 incrusted throughout its entire length ? Fig. 10 is not an ex- 

 ceptional example, there are others precisely similar, lying close 

 by it in the same section. If this be an instance of capricious 

 incrustation, we need hardly be surprised at it, when we consider 

 the singular way in which we sometimes find chlorite investing one 

 particular face of a quartz crystal in preference to the others. 

 Probably, however, there is no caprice in nature, and phenomena 



