120 Minm.KMiss ; 'nil-: c;i:oLor.v of tbAB statk. 



in large coherent masses, whilst the older is in comparatively small 



patches, in small sills permeating the intruded rocks (by lit-par-ltt 



injection) or represented by veins of pegmatite. 



In all the occurrences of the Idar granite bosses described 



above there is no sign of any laccolith: habit. 



No laooolites, do r p}ie junction exposures, no less than the 

 foliation or banding. J r , 



general shape of the masses of this rock, suggest 



a below-the-surface condition much resembling what is visible 

 above, that is simple boss-like or plug-like forms with more or less 

 vertical boundaries. There, is also an entire absence of any folia- 

 tion, or banded structure or streak mess, in these acid igneous rocks. 



To sum up, the field evidence points to a wide difference in age, 

 appearance and composition between the granite, micro-granite, 

 etc., of Idar type now being dealt with and those already treated 

 of under the name of vein aplite, etc. I am now of opinion that 

 thin veins and apophyses from the Idar granite masses are at least 

 extremely rare and but seldom appear. Exactly under what con- 

 ditions of intrusion these plug-like masses were, formed it is difficult 

 to say, but, whatever they were, they seem to have been unfavour- 

 able to the development of offshoots in the form of an interveined 

 plexus. The petrographical details which 1 shall now enumerate will 

 conclusively indicate the differences mentioned above between 

 the two granites. 



The Idar granite, granophyre and micro-granite is frequently a 



very handsome rock of from coarse to medium, 



Colour, texture and an( j ccasionallv fine, grain, in colours varying 

 general appearance. . , , . . , ' , 



from shades of grey, cream, salmon-pink and 



darker reddish and purple tints, mottled in the usual way by the 

 ferro-nwnesian minerals and occasionally diversified by moderately 

 large porphyritic crystals of felspar. 



From the specific gravities of 8 typical specimens it appears 

 that the lightest rock has a value of 2-607 



Specific gravity. ( ^ and the heaviest 2-767 ( 4 2 5 5 3 ), the mean 



of the eight being 2-674. 



The prevalence of orthoclase and microcline as the chief felspar 



ingredients places the Idar rock among the potash 



General mineral aranites with generally a leaning towards the 



composition. adamellite and monzonite subgroups in certain 



cases. The amount of quartz varies somewhat, but it is never absent. 



The dark minerals are biotite chiefly, and hornblende secondarily, 



