460 



PROF. W. S. EOTJLTON ON THE 



[Nov. 1904, 



spherulitic bodies, while these latter are sometimes arranged in 

 bunches radiating- from the wall of the amygdaloid towards its 

 centre. It is clear that the brecciation of the wall of the nodule 

 occurred before the infilling of this silica and brown spherulitic 

 matter, for they are arranged in concentric borders around the 

 angular and isolated fragments of the wall. 



Fig. 1 is a sketch of a nodule 7 inches across, collected by myself 

 from locality (433). There appear to be three generations, as it were, 



Fig. 1. — /Sketch of a nodule of complex structure, 

 measuring 7 inches across. 



A— Fibrous border. 



B= Quartz-amygdaloid. 



C= Quartz and brown spherulites. 



in the formation of this nodule. Fibrous borders have been formed 

 apparently around two small vesicles ; one nodule thus formed, 

 containing a more or less rounded amygdaloid, has become partly 

 enveloped by a larger, including an irregularly-stellate quartz- 

 amygdaloid, while all three have been enveloped in a dark-brown 

 fibrous layer, which forms the outer wall of the nodule. 



A specimen of slag given to me some years ago by Mr. H. T. Waller 

 is interesting in this connection, and seems to have some bearing 

 upon the origin of these pyromeridal and lithophysal structures. It 

 is a compound vesicle or lithophyse, 1% inches across, in a bluish- 

 grey glassy slag (fig. 2, p. 461). The main vesicle is surrounded by 

 roughly-concentric arcs of light-brown transparent glass, and be- 

 tween these glassy layers are crescentiform spaces. If this structure 

 occurred in a rhyolite, if the glass then devitrified, and brown fibro- 

 radiate microfelsitic matter developed in and around it, and the 



