558 W.CROSS, J.P. IDDINGS, L. V. PIRSSON, H.S. WASHINGTON 
From these it is seen that aluminum and calcium combine to 
form the orthosilicate, anorthite, in the presence of uncombined 
TiO,, so that the rule regarding the calculation of titanite and 
perovskite, rule 3(6), should be modified to read: Excess of TiO, 
over available FeO is combined with CaO in the ratio of CaO: 
TiO,::1:1 for titanite and perovskite, after the allotment of CaO to 
Al,O; for anorthite, rule 5(a), and according to the silica available, 
etc. 
Recent investigations in the Geophysical Laboratory of the 
Carnegie Institution in Washington have shown that the aker- 
manite compound is 3CaQO.2Si0,, not 4CaO.3Si0O, as formerly 
stated. This involves changes in the formulae for the calculation 
‘of normative akermanite as follows: 
For the reduction of diopside to akermanite 
= of the deficit of SiO, 
Y =molecules of akermanite (3CaO. 2Si0.). 
For the reduction of wollastonite to akermanite 
Y =deficit of SiO, 
Y =molecules of akermanite (3CaO. 2Si0.) 
If there is not sufficient wollastonite to satisfy the deficit of silica, 
recalculate the molecules of diopside and wollastonite so as to make 
akermanite, olivine, and diopside by means of the formulae 
2x ayt= =available SiO, 
x+3y =molecules of CaO 
x+zs =molecules of MgO+FeO 
Where x«=molecules of new diopside, y=molecules of akermanite, 
and z=molecules of olivine. 
Common errors im the calculation of norms.—Although clearly 
explained in the statements published regarding the method of 
calculating normative minerals in the Quantitative System and the 
method of determining rangs and subrangs, there often appears 
to be a misunderstanding in the minds of some petrographers as to 
the CaO and K,O and Na,O, which are to be used in determining 
the rang of a rock in Classes I, II, and III. It is a common error 
to use all the CaO and K,0++Na,O in the rock analysis in the ratio 
