116 THE PTROMEBIDES OF BOULAT BAT ( JERSEY). [Feb. 1 898. 



IV. Ptromerides with eelativelt large Quartz-enclositres. 



Figures (6-8) are given illustrating the relation existing between 

 nodules and the contained quartz-masses from Boulay Bay. 



It will be seen that they do not differ materially from those 

 already described by many authors. The accompanying diagrams, 

 however, show instances of quartz-enclosures from the harbour at 

 Boulay Bay which deserve a few words, on account of the rather 

 unusually elongated shape that they present. 



They occurred in the pyromeride-bearing rock which has been 

 above described as containing two constituents. Of these, one, 

 purplish and viscous, has formed the nodules ; the other, more fluid, 

 the investing ' matrix.' It is with the former that these elongated 

 quartz-masses are associated, a relation which seems significant as 

 to the condition and circumstances under which it has solidified. 



In these pyromerides typical contraction-cracks are often present, 

 but between these and the irregular, comparatively large quartz- 

 enclosures, often common also, are many intermediate shapes. 

 Thus the quartz, though long in proportion to its breadth, and with 

 crenated edges, seems too broad often for a contraction-crack, even 

 though a general correspondence obtains between it and the nodule. 

 The size of the latter seems of secondary importance : that is, the large 

 pyromerides do not necessarily contain more quartz than the smaller, 

 or vice versa. It is, however, sufficiently clear from the shape that 

 these are gas-vesicles, and that they are not due to any selective 

 action on the part of agents of decomposition ; and it is also clear 

 that, whatever force elongated the vesicles, it had ceased to operate 

 before the formation of the pyromerides, for these are not elongated. 

 Perhaps an explanation may be sought in the state of the rock 

 before solidification. The matter in which the viscous pyromeridal 

 material was placed was a more perfect glass. The former, with its 

 associated vesicles, drawn out into irregular flow-bands or left as 

 isolated clots by the action of flow in the latter, would, perhaps, 

 when the whole came to rest, be enabled to draw itself together — 

 like oil in water — assuming, with its enclosed vesicles (the pressure 

 in which would by this time have diminished), more spheroidal 

 outlines, those of the nodule naturally the more perfect. 



In conclusion I wish gratefully to express my thanks to Prof. 

 Bonney for ever-ready help, and for the kindness which he has 

 shown me during the preparation of these notes. 



EXPLANATION OF PLATE VII. 



Fig. 1. Part of cross-section of a fluxional band composed of pyromerides. The 

 figure shows the structure of a pyromeride, and includes a contraction- 

 crack concentric vrith the periphery, and showing recurved extre- 

 mities. Numerous brecciation-veins traverse the section. The centre 

 shows a somewhat different structure from the rest. In places it is 

 composed of ill-developed spherulites (not included in the figure). 

 Chff north of jetty, Boulay Bay. X 9^. 

 2. Pyromerides and enclosing perlitic rock, illustrating the difference in 

 the nature of the two. The irregular shape of the former, as de- 

 scribed in the text, is plainly shown. From beach in harbour, Boulay 

 Bay. X9i. 



