20 Notes on the Diabase Rocks 



examination. Such a distribution of the rock-forming con- 

 stituents — if the analysis be correct, and if I have correctly 

 interpreted the microscopic data — should close without a 

 remainder. 



In making this calculation I have availed myself of a 

 simple and effective method used by Professor Rosenbusch 

 in his valuable work, " Die Steiger Schiefer" 1877. Not 

 only is this method an extremely simple and effective one 

 for calculating rock analyses, but it also affords an easy 

 means of making a comparison of the ratios between the 

 totals of acids and of bases which form the rock as a whole. 

 To my mind it is preferable to the usual method of com- 

 paring the oxygen ratios. It affords what may be termed a 

 mineralogical view of the composition of the rock instead 

 of a merely chemical view. This is of great advantage 

 when considering the probable proportions in which the 

 various percentages are to be assigned to the constituent 

 minerals. I have annexed this calculation to the analysis, 

 and as it indicates a particular ratio for any one rock, 

 and therefore a ratio within certain limits for a kindred 

 group of rocks, I propose to speak of it shortly as the 

 indicative ratio. 



The method of calculation is based upon a comparative 

 distribution of the constituent percentages calculated as 

 molecules, between the minerals which are found to build 

 up the rock, and the distribution is made in accordance 

 with the most probable formula that can be assigned to the 

 mineral.* 



In calculating the molecular proportions of the percent- 

 ages I have used the old atomic weights as giving simple 

 numbers, and I have multiplied the results by 100, in order 

 to bring them better under review. 



* I have made use of the formulas given in Professor Groth's admirable 

 " Tabellarische ilbersicht der einfachen Mineralien" Braunschweig, 1879. 



