BY H. C. RICHARDS. 179 



VI. RELATIONSHIPS OF THE VOLCANIC 

 ROCKS TO ONE ANOTHER. 



Thirty-five complete chemical analyses of the volcanic 

 rocks are available. These, which may be regarded as repre- 

 sentative of the extruded material, have all been tabulated 

 earlier in this paper. For purposes of better comparison, 

 however, the accompanying table has been drawn up, and 

 all the analyses have been recalculated to 100 per cent, 

 after the subtraction of the water &c. 



In the list of rocks analysed, there are representatives 

 of the upper, middle and lower divisions, and also of the 

 alkaline and sub-alkaline types. An examination of the 

 table shoM's that the rocks range from acid to basic, and 

 the order followed is simply one of silica percentage in the 

 recalculated analyses. The numbers which the rocks are 

 here given are those used for the rocks in the variation 

 diagrams. 



Owing to the great extent of volcanic rocks, their 

 stratigraphical relationship to one another, the denudation 

 which has gone on, and the great difficulty of mapping-in 

 boundaries owing to the ruggedness of the country, it 

 is a matter of difficulty to accurately estimate the relative 

 volumes of the different outpourings. But as the im- 

 portant masses are all known and the average thicknesses 

 of the various divisions can be determined it is possible 

 to make an estimate as to the general ratios of the volumes 

 of the extrusions. 



This estimate used in conjunction with the chemical 

 analyses enables one to make an approximation to the mean 

 average composition of the extruded material. 



It is believed that all the volcanic rocks of the area are 

 magmatically related, and that the parent magma from 

 which they have been derived had the composition of a 

 nearly normal andesite. 



Average Composition of the Volcanic Rocks of the 

 Upper Divisio)i. 



Fourteen analyses of lavas known definitely to belong 

 to this division have been used. The rocks analysed are 



