94 



PEOCEEDIXGS OP THE GEOLOGICAL SOCIETY. 



An examination of Prof. Danbree's results suggests tliat the devi- 

 trifj'ing action has been more universal and simultaneous throughout 

 the mass than in the above-described cases of " dry heat" or " cooling " 

 devitrification. It is true that the surfaces of the mass have, in this 

 case also, produced modifications ; but spherulites appear to have 

 started almost simultaneously from many independent centres, so as 

 to form a crowded mass, interlocking with irregular outlines, instead 

 of a number of large spherulites, Trhich. if they come into contact, 

 are parted by more uniform surfaces. The formation also of 

 innumerable microliths throughout the whole mass of the glass is not 

 weU paralleled in the instances of " dry-heat '' devitrification. K'ow 

 on examining cases where we may reasonably conclude that a devi- 

 trification (in the strict sense of the word) has occurred in nature, 

 we are struck with certain structural peculiarities. We may. I think, 

 assume that the existence of a perlitic structure in a rock is an indi- 

 cation that it has once been a true glass. Isolated spherulites and a 

 well-marked banded structure are, I believe, also presumptions in 

 favour of the same : though in the latter case portions may have 

 assumed a crystalline condition in cooling. If, then, we examine 

 slides of such rocks as the devitrified perlitic lavas of the TVrekin 

 district, we observe that the secondary structure presents certain 

 peculiarities. i!^ot seldom it bears a defijiite relation to the cracks 

 by which the perlitic mass is traversed, ' a thing not surprising, 

 because these cracks, as pointed out some years since by myself, and 

 as indicated to you during the present year from another point of 

 view by Mr. Eutley, may have from the very first been connected 

 with pressures or strains of some kind, and this distiu-bance of equi- 

 librium could scarcely fail to tell when crystallization commenced. 

 There are, indeed, instances to be found where the depolarization- 

 phenomena ordinarily seen in a colloid body subject to strain seem to 

 have been rendered permanent. The devitrification-structures in these 

 perlitic rocks difPer much from those which I have observed in any case 

 where there was a reasonable probability of their being the result of 

 the original cooling. It is difficult to express it in words, without 

 entering into lengthy and minute details unfitted for the present 

 occasion ; but I may epitomize them thus : — the slide throughout 

 exhibits a peculiar confusedly crystalline structure, the individualized 

 minerals sometimes being of extreme minutenes:-. The ground-mass 

 appears to be composed of a mixture of quartz and felspar ; but it is 

 exceedingly difficult to say which has been first to consolidate, some- 

 timesthe one, sometimes the other, appearing to have had the mastery. 

 Xow and then a felspar crystal exhibits a rectilinear boundary, 

 but very commonly it appears to granulate into the quartz, and 

 sometimes the felspathic mineral (I am doubtful whether it is a true 

 felspar) resembles a kind of residuum or '• sediment," left unused 

 when the quartz grain had formed. The latter mineral frequently 

 occurs in little groups of moderately distinct, though crowded ciystals, 

 as may be observed in some cases of chalcedonic formation in veins 

 and cavities. Close intercrystallizations of the quartz and felspars, 

 leading to all kinds of imperfect =pherulitic, micrographic, and den- 



