434 MR. J. PARKINSON ON DEVITRIFICATION. [Nov. I903, 



however, it is present in such quantity as to render the slice almost 

 opaque at that point, the grains tend to lose distinctness of outline. 



(b) — Those in which secondary structures have been superposed 

 on primary. 



Of these the most familiar example is the irregular mosaic of 

 grains — referred to as ' patchy devitrification.' l 



The patches which become apparent between crossed nicols appear 

 to be formed by the crystallization, or recrystallization, of a residual 

 mineral. In the case of a spherulite, this is the substance left over 

 after the recrystallization of the radial fibres of felspar. The 

 stresses which acted on the rock before solidification affect it after 

 solidification ; for the ' patches ' are elongated, radially in the case 

 of a spherulite, parallel to the direction of flow in a rhyolite show- 

 ing marked fluxion-structure. In the Boulay-Bay nodules, the 

 dimensions and definition of the ' patches ' vary considerably in the 

 same example, and at times appear better formed near the periphery. 



Many of the Boulay-Bay nodules exhibit a kind of segregation, 

 no doubt primary, which gives rise to a sponge-like network, the 

 interspaces being occupied by more translucent areas, as apparent 

 in ordinary light as between crossed nicols. Under the latter con- 

 ditions each of these oval spaces is occupied by material with 

 definite and uniform polarization, differing but little from an isolated 

 individual of the more common type of ' patchy devitrification.' 



In regard to the relation between the ' patchy devitrification ' 

 and the granular mosaic produced by this secondary devitrification 

 of a perlitic rock, see Part II, p. 440, by Prof. Bonney, to whom I 

 am greatly indebted for help and suggestions made from time to 

 time, the results of which are embodied in the foregoing pages. 



I Excluding spherulites, we may make the following summary : — 



Primary Structures. Secondary Structures. 



(a) Glass containing no primary Granular secondary devitrification 



devitrifications. Homogeneous uniform over a small area (that 



throughout. is, over a single slide). 



(b) Glass containing no primary de- Granular secondary devitrification not 



vitrifications. Not homogeneous uniform over a small area. Vari- 



throughout. ability in composition traceable 



in a single slide. 



(c) Rhyolite containing some primary Secondary structures /; and d com- 



devitrifications, as, for example, bined. 



bands or patches of eutectic, and 

 crystallizations (gnarled struc- 

 ture and the like) representing 

 separation of a constituent in 

 excess ; and some original glass. 



(d) Rhyolite not glassy in any part. Secondary superposed on primary de- 



vitrification, of which f patchy 

 devitrification ' is the type. 



1 For a discussion on this structure see F. R. C. Reed, Quart. Journ. Geol. 

 Soc. vol. li (1895) p. 165. [A slightly-different explanation of this structure is 

 offered in Part II of the present paper, p. 441.— T. G. B.] 



