Prof. Sedgwick on the Structure of large Mineral Masses. 467 



spheroidal concretions are not exceptions to, but examples of, the very changes 

 I am attempting- to describe. 



(4.) Rocks of Globular Structure subordinate to the Old Slate Formation of 

 North JValcs. — This structure may be often seen in the mountains of Caer- 

 narvonshire and Merionethshire^ yet I do not remember to have seen a single 

 good example of it in the corresponding formations of the North of En- 

 gland. Tabular masses of porphyry, compact felspar, hornstone, &c. alternate 

 with, and partake of, all the great flexures of the true slate rocks of North 

 Wales. Some of these have, I doubt not, been poured out, in a state of igneous 

 fusion, at many successive times during the long period in which the whole 

 slate series was deposited. Others, on the contrary, seem to have been de- 

 posited as a part of the schistose series, and to have been since changed into 

 their present forms by crystalline forces, pushed, probably, into activity by a 

 high temperature. It is in this latter class that I arrange most of the globular 

 rocks I am now noticing. 



In the compact felspathic slates we may often see balls of nearly pure quartz. 

 These balls are sometimes as much as three or four inches in diameter, and 

 are arranged with considerable regularity. Their centres are generally com- 

 pact, but sometimes hollow, exhibiting a tendency to an agate-like structure; 

 and the circumference of these balls is almost always ill defined, gradually 

 blending itself with the prevailing mass of the rock. This, if I mistake not, is 

 one of the modifications produced, in sedimentary rocks, of which the original 

 structure (when they are examined on a great scale) is sometimes not entirely 

 obliterated. 



The proportion of quartz varies extremely in other great globular masses, 

 and sometimes it forms nearly the whole rock : in such cases they might be 

 at first sight mistaken for great white quartzose conglomerates. On fracture, 

 the balls are found to be more or less crystalline; not unusually they are hollow, 

 and partially agatized ; and, on examination, we find them held together by 

 a rude cement, in which we may sometimes detect a granular structure. Such 

 masses as these could not, I think, be of direct igneous origin. I believe them 

 to be the result of changes produced by slow long-continued chemical action 

 on beds of old stratified sandstone. 



These two examples of changed structure differ from all those above enu- 

 merated, in as much as they seem to require a high temperature for their comple- 

 tion, and are probably, in all cases, nearly associated with igneous rocks: and if 

 any of these globular masses should appear to have been in a state of igneous 

 fusion, they ought in that case to be removed into another class, and arranged 

 with the orbicular granite of Corsica and other concretionary trappean rocks. 



