92 ALFRED C. LANE 



The general rules for computing the norm of a rock can be 

 simplified if made applicable only to this division. 



2. Basic. Silica percentage, 0.58 — , bounded, as shown on 

 diagram perhaps, or by some other line expressing the fact 

 that the eutectic is not melilite, but augite. Eutectic ratio, Ca : 

 (Mg: Fe):Si02 : : I : I : 2. 



Basalts, gabbros, peridotites, etc. Computing the place of 

 such rocks in a quantitative system is quite simple. All the 

 alumina can be combined with potash, soda, and sufficient 

 lime and counted salic, everything else femic, so that the ratio 

 sal : fem is quickly found, and the ratio of soda to potash and 

 alkalis to lime are found incidentally. 



3. Alkaline. All other rocks, which obviously should also 

 be subdivided, perhaps, as shown in the diagram, into a femic 

 and salic group, and farther yet. 



What ought to be done is carefully to study the whole field, 

 with due regard to magmatic splitting, watching the last crystal- 

 lization, and determine as nearly as may be what are the eutectic 

 ratios in the silicate magmas. Then the work should be experi- 

 mentally verified, in the new geological laboratory that I hope 

 we are to have. Then, finally, we shall no longer have to envy 

 the metallographist,^ who measures the areas of micropegmatite 

 (eutectic) copper and copper oxide, and areas of solid copper, 

 and says: "So much eutectic with 3.45 per cent. Cu^O, and so 

 much plain copper: there must be just so much copper and so 

 much oxygen." 



In such study it is the last-formed minerals, with a little of 

 nearly everything in their molecule, which show by their compo- 

 sition the eutectic proportions of the different constituents. 



Since writing the above I have noticed in the Beilage Band 

 XVII of the Neues Jahrbuch, p. 516, and especially pp. 546-64, 

 an article by Schweig, and in the Ceiitralblatt, 1903, p. 605, a 

 note by Linck on a series of experiments exactly along the line 



' HoFMAN, Green, and Yerxa, " A Laboratory Study of the Stages in the Refin- 

 ing of Copper," Transactions of the American Institute of Mining Engineers, October, 

 1903. 



