190 ME. G. A. J. COLE ON THE ALTEEATION OP 



EXPLANATION OF PLATE IX. 



•■ 



[Where the degree of enlargement of the objects drawn is expressed by a 

 fraction, the numerator represents the magnifying-power of the objective with 

 which they were viewed.] 



Pig. 1. Section of end of ramification of infilled cavity in large spherulite, 

 pitchstone of Zwickau, showing its connexion with a crack. The lines 

 of flow are cut through by the cavity, which contains chalcedony and 

 calcite. A portion of the glassy matrix is seen below. X-%°. 



2. Section of coarsely spherulitic rhyolite, Wrockwardine (Wrekin) area, 



showing difierence in width of cracks as they pass from the matrix 

 into the spherulitic matter. The matrix is seen to be perlitic. X \°-. 



3. Large compressed spherulite, broken across, from rhyolite of Digoed, 



N. Wales. Extensive development of quartz and chlorite has taken 

 place, and the residual spherulitic matter forms but a thin encircling 

 wall. Natural size. 



4. Section of two spherulites from rhyolite of Digoed, N. Wales, showing 



zones of granular secondary quartz, with lines of liquid enclosures 

 passing continuously from grain to grain . X 2. 



5. Portion of the quartz of the above with lines of liquid-enclosures. 



vx 1 00 



6. Section of compressed spherulite in rhyolite of Conway Mountain, 



showing alternate concentric layers composed of a black product 

 of alteration. x2. 



7. Section of black alteration-product in centre of spherulite, rhyolite of 



Conway Mountain, showing development of tufts of crystallites 

 within it. x H-- 



Discussion-. 



The Peesiden^t said that Mr. Cole had attacked a very puzzling 

 problem and had given an explanation which appeared to be con- 

 sistent with the whole of the phenomena. 



Prof. Bonney said that the subject was not only difficult to 

 elucidate, but also difficult to discuss. He did not understand the 

 formation of the supposed alteration-products from the white sphe- 

 rulitic rock. Again, he had noticed in one of the microscopic slides 

 a crystal of iron-oxide close to the edge of a cavity. If the cavity 

 was due to decomposition, why was not the crystal affected? 

 Another difficulty was to conceive what agent could have produced 

 the decomposition, what acid could have acted on one part of the 

 rock, leaving other parts quite unaffected. He was rather inclined 

 to attribute this kind of spheroidal structure to cracks forming 

 around vesicles. The radial or true spherulitic structure was, in 

 some cases, connected with, but subsequent in origin to, the formation 

 of the cavity. He, however, congratulated Mr. Cole on the ingenuity 

 of his views and on the lucid manner in which he had explained 

 them to the Society. 



Mr. Eutlet said that he was unable to accept any view hitherto 

 offered as to the origin of these bodies. He showed that there were 

 several different kinds of spherulites, and gave examples. He in- 

 stanced the concretions in the Magnesian Limestone of Durham as 

 affording structure which sometimes apparently bore a certain 



