THE ALTERATION OF COARSELY SPKERTJLITTC ROCKS. 183 



14. On the Alteration of Coarsely Sphertjlitic Rocks. By 

 Grenville a. J. Cole, Esq., P.G.S. (Read March 10, 1886.) 



[Plate EX.] 



As has been previously stated*, all the phenomena that accompany 

 spherulitic structure on an ordinary scale appear reproduced among 

 lavas that may be called coarsely spherulitic. Ill-defined segrega- 

 tions, differing little from the matrix, the fluidal and banded struc- 

 tures of the rock running indifferently through both, are found 

 measuring an inch or two in diameter, equally with forms of marked 

 radial or concentric character. The scale on which certain processes 

 of decomposition and reconstruction, especially those that affect 

 differently the spherulite and the matrix, are revealed in these bold 

 examples, and the interest attaching to their effect upon the ulti- 

 mate character of the mass, have led me to bring together a few 

 observations on the alteration of coarsely spherulitic rocks. Some 

 of the finest instances, moreover, may be found in our own British 

 lava-flows of Pala30zoic age, and with these the field-worker is not 

 unfrequently called upon to deal. 



The well-known black pitchstone of Planitz, near Zwickau in 

 Saxony, which is of Permian age, contains angular aggregates of 

 calcite and chalcedony enclosed in large brown nodides. These 

 remarkable occurrences were referred to by Cotta as early as 1849t, 

 and his figure was copied by Delesse in a comprehensive j)aper on 

 " Les Roches globuleuses " J. Cotta considered the nodules sur- 

 rounding the chalcedony as inclusions of an adjoining porphyry, like 

 those in the pitchstone of Spechthausen ; but the microscope proves 

 them to be in reality large "brown spheruHtes, often 3 centims. in 

 diameter, and differing only slightly from the glass. Why the 

 agents of alteration should have attacked them in so marked a 

 degree is indeed a puzzling matter, which will be best solved by a 

 study of the " Lithophysen " of more recent lavas ; but there can be 

 little doubt that these spherules were once solid to the core. Since 

 they have no internal radial or concentric structure, the decompo- 

 sition has hollowed them out irregularly, or has started along and 

 spread from any cracks that might occur. The relation of the shape 

 of the cavity so formed to these preexisting cracks may often be 

 seen at the angular ends of its branches. The hollow has ultimately 

 been filled with chalcedony or with calcite, which may also be found 

 in cracks throughout the rock. There is no sign of a central mineral 

 nucleus from which this material might have been derived, since the 

 lines of flow throughout the nodule do not bend round as when an 

 obstacle is present, but may be connected up with one another 

 straight across the branchings of the secondary mass. The same 

 evidence strongly opposes the supposition of the former existence of 



* Q. J. G. S. vol. xli. pp. 165, 166. 



t Leitfaden und Vademecum der Geognosie, p. 76. 



X M^moires de la Soc. geol. de France, 2ine serie, tome iv. p. 315. 



