tut 179 



the mineral he trie* to find ii in tiht, and if 

 compares the new habitat with thoae previously observed 



further ajttume that he discovers it always to lie 

 in the same long bladcd plates or prisms, with the 

 issof liffn group of minerals, and in the same or a 

 km.l of rock. This (act established would be one of 

 hs^h importance in any attempt to work out the geologi- 

 cal structure of the district Massive hornblende, acti- 

 nolitc, garnets, hlonic, and different micas may all be 

 found to characterise particular bands of rock. In many 



regions, however, no such special mineral cones are 

 to be seen. The rocks present a singular monotony of 

 character, or their abundant minerals are not confined 

 to special horizons, so that if their order is to be deter- 



i, it must be done by some other method than that 

 furnished by mineral evidence. 



.- !'. :: u : - re the rocks furnish no specially prominent 

 mineral zones, they often present in themselves great 

 varieties of composition, structure, and texture. The 

 observer will duly note these characters with the view of 

 ascertaining whether he has to deal with true Archaean 

 massm or with a younger series of altered sedimentary 

 formations. He discovers, let us suppose, a coarse- 

 grained mass of gneiss with large crystals of pink ortho 

 clase so peculiar a rock that even small fragments can 

 readily be recognised. After not thotogical char- 



acters, he proceeds to investigate its surroundings. He 

 finds that though, looked at in hand-specimens, it seems 

 strm tureless, like a coarsely crystallised granite, yet that 

 a distinct foliated structure exists throughout its mass, 

 ami that parallel with this foliation there occur, on either 



