CHAPTER X. 



MICA-TRAPS AND RELATED ROCKS. 



In this chapter we shall refer to certain rocks of somewhat except- 

 ional composition. These rocks do not fall readily into any scheme of 

 classification based on Bunsen's law, as will be seen by an examination 

 of the list of analyses quoted later on. In Britain they occur abundantly 

 in two principal localities : in the Kendal and Sedbergh districts of 

 Westmoreland and north-western Yorkshire and in the West of En<*- 

 land, more especially Cornwall. The rocks of the former district have 

 been described by Messrs. BONNEY & HouoriTON^ ; those of the latter 

 by Mr. COLLINS. As matters stand at present there appears to be a 

 difference in the age of the rocks in these two localities ; those of the 

 North of England occur as dykes in the lower Palaeozoic rocks and are 

 never known to enter Carboniferous strata ; those of the West of England 

 on the other hand occur in rocks as high as the Culm Measures. 



The rocks in question always occur as dykes or veins which are 

 often very narrow and difficult to trace for any considerable distance. 

 They are generally much decomposed and frequently show a tendency 

 to spheroidal weathering. Carbonates are generally present in consider- 

 able quantity. A brown mica is also usually present in considerable 

 quantity and sometimes gives the rocks a decided porphyritic character. 

 Felspar rarely occurs as a porphyritic constituent and in this respect 

 these rocks differ from the porphyrites and porphyritic diorites. The 

 investigation of these rocks is attended with great difficulty in consequence 

 of the amount of alteration which has taken place. They are never found 

 in a perfectly fresh condition. By selecting a number of the best 

 preserved specimens it is possible to establish something like a classi- 

 fication of the group, but in working in the field rocks are frequently 

 met with which it is impossible to refer to their proper position in such 

 a classification. A vague term like trap or mica-trap is the only one 

 that can be used in such cases. The following names have been applied 

 to members of this group : minette, kersantite, kersanton, lamprophyre, 

 mica-diabase, minette-felsite, kersantite-porphyrite, mica-porphyrite, 

 mica-diorite, mica-syenite. ROSENBUSCH has recently proposed two new 

 names, vogesite and camptonite, for certain rocks allied to the mica-traps. 



Minette. This is a local name used by miners in the Vosges for 



(1) Q.J.G.S., Vol. XXXV. (1879), p. 165. 



(2) Jour. Roy. lust., Cornwall, Part II., Vol. VIII. (1884). 



