82 Washington — Pyroxenite and Hornhlendite in Brazil. 



manganese is extremely high for this constituent. Indeed this 

 rock carries the highest amount of MnO of any igneous rock 

 known to me in which the figures for this constituent are reli- 

 able. This feature will be discussed later. 



Classification. — The norms of these rocks are as follows, the 

 letters corresponding to those of the analyses : 



A B C D 



Or 1-11 7'23 0-56 1-67 



Ab 11-00 3-67 9-43 11.00 



An 15-01 14-73 25'85 20'02 



Ne _.- 4-26 9-09 



Di 31-04 24-72 25-00 30*85 



Hy 3-00 



01... 28-76 34-51 23'78 24-19 



Mt 6-50 1-39 7*19 9-05 



II 1*37 2-43 2-28 1*98 



Ap 1-01 



From these figures it is seen that A and B fall in rossweinose 

 (IY. 1.3.2.2) and C and D in uvaldose (IV. 2.3.2.2), the complete 

 symbols in each case being given with the analyses. The differ- 

 ences are, however, not great, they all contain so much norma- 

 tive feldspar as to be intermediate toward the salfemane class. 

 A and B carry such small amounts of ores as to fall in order 1 

 (although intermediate toward 2), while C and D carry so 

 much that they are over the line in order 2. As regards the 

 other divisions they are all substantially identical. 



Mode.— The Maracas rock, being coarse-grained, simple in 

 composition, and very fresh, measurement of the mode by 

 Kosiwal's method is very easy. A large series of traverses 

 gave the following results in percentage by weight : 



Hornblende 91-3 



Olivine 3*6 



Magnetite _ 5*1 



100-0 



On comparing this with the norm, it is evident that the large 

 amount of normative feldspar and nephelite molecules have 

 entirely entered the hornblende, as is true of other hornblendites. 

 In this it differs modally from the chemically similar kylite (B), 

 which is described as a feldspathic peridotite with some augite, 

 so that its mode is more nearly normative. The difference is 

 evidently due to differing conditions during crystallization, and 

 will be discussed later. 



The simplicity of the mode, and the very small amounts of 

 olivine and magnetite present, permit the calculation of the 

 composition of the hornblende with a very fair degree of 

 accuracy. For this the MgO and FeO of the olivine are 



