COARSELY SPHEEULITIC EOCKS. 



187 



\ 



resemble fragments of included slate, still showing a relation to tlie 

 curving zones of the great spherules. Its origin is, however, much 

 better indicated by the Conway specimens. 



With a specific gravity of 2-77 *, a hardness of about 2*5, and all 

 the appearance of a metamorphosed clay, it became interesting to 

 compare this decomposition- product with the parent rock. I suspect 

 that the composition of both varies somewhat in different specimens, 

 especially in the matter of silica, which is so largely accumulated 

 in the cavities of the rock. The portion of the white matrix and 

 spheroidal matter chosen was, so far as could be seen, free from 

 secondary quartz ; but the high percentage of silica shown probably 

 over-represents its original proportion in the lava-flow. 



White spherulitic 

 rock of Digoed. 



iSiO, 83-08 



AiPg 10-25 



Fe.p^ trace 



CaO '2Q 



MgO , . -09 



K,0 1-78 



'Naft 3-58 



Loss on ignition . . -74 



99-78 99-91 100-2 



I have placed in a third column the average of six analyses of 

 pinite quoted by Ilammelsbcrg f. It will be seen that the figures 

 of the second column generally fall within the extremes made use of 

 in calculating those of column III., the main difference being in the 

 proportions of the alkalies. 



The alteration-product of Digoed (and, by inference, the black 

 material of the other ancient rhyolites) seems, then, to be related to 

 the substances of the pinite group, and has probably as little claim 

 as they to be regarded as a mineral species. 



Under the higher powers of the microscope, moreover, it is seen 

 to be composed of particles of various kinds. In one nodule from 

 Digoed it has given rise to minute crystalline tufts and bunches 

 of a uniform character:!:, interspersed with secondary quartz (PI. IX. 

 fig. 7) ; but a large section of the fragment used for the analysis 

 consists mainly of transparent angular grains of a very pale blue-green 



* The specific gravity of the white matrix rises as high as 2-67, but is 

 variable through included quartz. 



t Handbuch der Mineralchemie (2nd edit.), Band ii. p. 656. 



I The substance discussed by Kalkowsky (" Untersuch. von Felsiten und 

 Pechsteinen Sachsens," Tscherm. Mittheil. 1874, p. 46) may perhaps be of a 

 'similar nature. 



n. 



III. 





Average of six 





analyses of pinite 





(the extreme per- 



Black layer in 



centages made U3§ 



large spherulite, 



of are placed in 



Digoed. 



brackets). 



50-75 



47-8 (44-7-54-6) 



28-34 



29-9 (23-6-32-4) 



3-63 



5-7 ( 0-9-10-2) 



2-57 



•7 ( 0-0- 2-4) 



1-85 



1-8 ( 0-0- 3-4) 



6-21 



8-7 ( 6-5-11-2) 



2-19 



•5 ( 0-0- 1-1) 



4-37 



5-1 ( 1-2- 7t8) 



