Vol. 54.] or EOULAY BAY (JEESEY), 105 



also to observe that the pjTomerides occur frequently in seams, and 

 that in adjacent bands in the rock there may be developed little 

 rounded knobs or lumps, even though the band itself has not been 

 converted into pyromerides, as one must believe to have happened in 

 the instances which have been just described. 



As a type of the nodules which bear no quartz-mass in their 

 interior (the ' globules normaux ' of Delesse) the following instance 

 may be taken. The length slightly exceeds 1| inch, the breadth 

 1^ inch, so that, as is very often the case, the pyromeride is oval ^ 

 and not circular in longitudinal section. The external surface has 

 inequalities and mammillations ; and, when fractured, there is no 

 trace of a radial structure, a condition which, though very usual, is 

 not invariable. The surface is of a reddish-brown, rather chocolate 

 colour, with the exception of an irregular area, rather lighter, sug- 

 gestive of the streaky or banded rock of the immediate vicinity. 



In a thin section, as might be expected, the pyromeride is rather 

 opaque. Over the greater part, a process perhaps best described as 

 a ' settling-down,' seems to have taken place, so as to produce an 

 irregular sorting-out of the various constituents, an arrangement 

 more marked in some parts of the slide than in others. Indepen- 

 dently of this, there is a very slight radial structure. A ferruginous 

 constituent in a finely-divided state is present in the slide in some 

 quantity, and this in places, having been largely excluded by the 

 segregatory process, forms a black meshwork which is brick-red by 

 reflected light. The part mentioned as being lighter macroscop- 

 ically is strikingly marked off in thin section from the rest. It is 

 quite clear, shows finely fibrous radial structure between the two 

 nicols, and contains a few quartz-grains. 



An equally common type is purple rather than red, and shows a 

 more uniform structure in a thin section : the radial growth, always 

 faint, being perhaps slightly more pronounced towards the exterior. 

 The type of depolarization is decidedly characteristic.^ It is 

 distinguished by the patchy mosaic into which the field is 

 resolvable ; each constituent area, though often extremely irregular, 

 polarizing separately. Occasionally a cryptocrystalline ground- 

 mass is interposed, in various degrees, between the areas of 

 depolarization, resembling the structure of a rock from the 

 Fishguard district described by Mr. Cowper Reed.^ Frequently, 

 clear and uniformly polarizing patches are seen in a more opaque 

 fibrous setting, which shows dull golden yellow when the fibres are 

 inclined at an angle of 45° to the vibration-planes of the crossed 

 nicols. Typically, hoAvever, no indication of the presence of a 

 mosaic can be seen in ordinary light ; and, when developed best, the 



^ The foregoing remarks are not intended to suggest that pressure may not 

 be responsible for the oval form of many nodules. See Quart. Journ. Geol. 

 Soc. vol. xxxviii (1882) p. 289. 



^ See Quart. Journ. Geol. Soc. vol. 11 (1895) p. 149 ; also many other 

 references contained in Mr. Eeed's paper. 



^ It is also seen occasionally in therhyolite itself, as, for example, near Bonne 

 Nuit Bay. 



