84 Scientific Proceedings, Royal Dublin Society. 



constitutes roughly between 75 and 80 per cent, of the rock. The 

 underlying assumption is that the relative extents of areas taken 

 all round are proportional to the relative abundance of the consti- 

 tuents ; an assumption undoubtedly justified unless we are 

 dealing with some special rock-structure. Thus it must be noticed 

 that the mica in some granites may be present in fairly parallel 

 plates. If these appear edge on in the field an allowance must be 

 made or the quantity of this constituent would be under-estimated. 

 This method is very rapid ; and a rough estimate of the chemical 

 composition of the rock may be founded on it if the minerals are 

 definite in kind. 



