INTRUSIVE ROCKS IN THE ARCHAEAN AREA. 169 



It is clear that this dyke belongs to a later geological period than 

 do the quartz-runs of the Harapanahalli series. The dyke rock is a 

 medium-grained diorite. At both intersections and for more consider- 

 able distance from both, the diorite is so full of torn off fragments of the 

 quartz that the rock constitutes a very bold and remarkable breccia 

 on a large scale. The enclosed fragments of quartz are, as a rule, of 

 medium size, but often form quite half of the mass seen. 



To the westward of the intersection with the quartz-run the dyke 

 is seen to contain great numbers of included fragments of a gneissic 

 rock. 



There is a very great similarity in the appearance of the dykes, 

 Very few quarries in an( * on ly a * ew °^ er varying features of suffi- 

 the trap dykes. cient importance to be noticeable. A very much 



closer examination than they received from me en passant might 

 very probably detect special points of interest. Id the very great 

 majority of cases the true nature of the rock is not exposed in 

 quarries, and where the rock occurs in rounded masses without 

 joints giving rise to angles, it was often impossible to penetrate 

 the external weathered crust without having recourse to blasting, for 

 which neither time nor apparatus was available. Sundry hammer 

 handles shivered in attempts to obtain hand specimens from unquar- 

 ried dykes. The dykes deserving of special notice on account of 

 peculiarities of composition or of structure are enumerated below. 



No cases of columnar cleavage of the rock were met with, nor 

 any in which the injection of the trap rock had 



cle^vage ea seen. C ° 1Umnar had an 7 visibIe effect on the rock traversed. Ac- 

 cessory minerals are of extreme rarity, pistacite, 

 red orthoclase and marcasite excepted, and the examples of the 

 two latter are very few'and far between. Of the red felspar crys- 

 tals the best examples were seen in a dyke in the Alur taluq, 

 4 miles northward from Alur town. Here the dark black trap is tra- 

 versed along planes of jointing by deep salmon-coloured, almost 

 crimson felspathic or felspatho-epidotic veins rarely, as much as 

 quarter inch in thickness, and it is along these that a few minute 



( 169 ) 



