550 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSM1LCH. 



shown in column I below. Adding to this 55 per cent, of 

 SiO s and 34 per cent, of OaO, and reducing to percentages 

 again, we have the composition shown in column II : — 



I. 



II. 



III. 



Si0 2 38-2 



49*4 



49*7 



A1 2 0> 12*8 



6'7 



5*8 



Fe 3 Oa andFeO 16*3 



8-6 



10-1 



MgO 28*0 



14'8 



13*0 



OaO 4-0 



20*1 



20*8 



Na 2 and K 2 "7 



•4 



•7 



100-0 lOO'O ioo-o 



and this compares fairly closely with the composition of 

 the augite, shown in column III. Thus for the silica and 

 lime together 89 per cent, must be added to the dehydrated 

 chlorite to give us the destroyed augite. Add to this the 

 existing augite, and we have the original total, thus : — 



5*0 x -84 x 1*89 = 7*9 + 34*2 = 42*1 = total augite. 



In restoring decomposed felspar the space now occupied 

 by analcite is to be reckoned as filled with felspar, say, on 

 the average an andesine (sp. gr. 2.65), so what has been 

 reckoned as analcite is converted to felspar by multiplying 

 by the ratio of their specific gravities, thus : — 



The quantity of felspar lost and now replaced by serpen- 

 tine and chlorite is found by calculating the expansion of 

 the olivine in passing to serpentine, and the contraction of 

 the augite in changing to chlorite and counting the balance, 

 which is always one of expansion if there be an appreciable 

 quantity of olivine present, as the volume of felspar replaced 

 by serpentine and chlorite. The several volumes are found 

 thus : — 



