14-3 



CHAPTER VIII. 



ON THE VARIOUS MODES IN WHICH THE GRANITIC AND 

 SCHISTOSE ROCKS ARE ASSOCIATED TOGETHER. 



The occurrence of granitic rocks !n the primary schists in the form of beds or 

 courses in irregular bunches or masses in Cornwall, Scotland, Ire- 

 land, and Norway. Portions of these schists also contained in granite 

 in Cornwall, in various parts of Scotland, and in France. The primary 

 strata traversed by granite-veins in Cornwall and in Scotland. Sum- 

 mary of the various appearances presented by these veins. 



HAVING detailed in the preceding chapter the nature and 

 position of the schistose rocks at their point of contact with 

 granite, it is now proposed to consider how these stratified 

 and unstratified rocks comport themselves, when more in- 

 timately intermixed in the form of beds, dykes, veins, and 

 detached portions of various irregular shapes and dimen- 

 sions. 



Some of these circumstances have been necessarily an- 

 ticipated, more or less, in the foregoing descriptions ; more 

 especially the occurrence of large and regular masses of 

 granitic rocks in the primary slates, as those of Cornwall, and 

 analogous examples in other countries, of which those of 

 Ireland, recorded by Mr. Weaver, are most minutely de- 

 scribed. These granitic beds, commonly known in Cornwall 

 by the name of elvan-courses, are sometimes so numerous, and 

 so regularly placed in the slate, that they appear to alternate 

 therewith; and, indeed, this mode of association is more 

 regular and persistent than is often to be met with among the 

 different kinds of primary slates ; for these pass into each other, 

 at various points, through such frequent and almost impercept- 

 ible gradations, that the same order cannot be detected. 

 These elvan-courses, however, also exhibit great irregularities, if 

 we descend from the general outline to a minute examination 



