ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 295 
erate to rather low silica, though it is worthy of note that the highest 
figures recorded for it are in some rocks of Wyoming which are low 
in lime and extraordinarily high in potassium and barium. Being 
but rarely a constituent of silicate minerals, decisive evidence from 
this side is wanting, though it occurs with lime in some heulandite 
and brewsterite. 
Barium is another element which the analyses of the Washington 
chemists showed to be widely distributed, and almost invariably in 
decidedly greater amounts than strontium. It is now often deter- 
mined by analyses of superior quality, and in a recent study, embrac- 
ing the rocks of Italy, the United States, and New South Wales, I 
have shown that it is specially prone to occur in potassic rocks, some- 
times when the potash is accompanied by considerable lime, but that 
it is rarely met with in notable amount in decidedly sodic or calcic 
rocks. Neither the amount of silica nor the relative proportions of 
iron and magnesia appears to be a determining factor of much im- 
portance. This association of barium and potassium in igneous rocks 
is in harmony with the mineralogical evidence. Barium is a frequent 
minor constituent in potassium minerals, as orthoclase, muscovite, 
and biotite, while potassium accompanies barium in hyalophane and 
harmotome. On the other hand, barium is not reported in analyses 
of sodium minerals, but occurs in small amounts in the calcium zeo- 
lites, brewsterite, and phillipsite. Barium also seems-to tend to asso- 
ciate with manganese, as shown by its common occurrence in psilo- 
melane and the occurrence of minerals of the two metals in certain 
mines. 
Boron is seldom or never mentioned in rock analyses, chiefly 
because of the complexity and difficulty of its exact determination, 
especially in very small amounts. But it is not infrequently present 
in weighable amounts in granites and pegmatites, chiefly as a con- 
stituent of tourmaline. The few analytical data that we have of 
such tourmaline-bearing rocks are not decisive, but boron does not 
appear to have very decided preferences for either soda or potash. 
Its associations in minerals are likewise not strongly marked, but 
among the silicates calcium is the basic element which most fre- 
quently accompanies it, and soda is more commonly met with in 
boron-bearing minerals than is potash. Boron is commonly regarded 
as one of the pneumatolytic elements. 
Cerium, yttrium, and the other metals of the so-called “ rare 
earths.” as well as ¢horiwm and uranium, are only rarely deter- 
mined in rock analysis. Minerals containing them are commonly 
associated with acid pegmatites, which, judging from occurrences in 
Norway, Greenland, and elsewhere, are most apt to be sodic, though 
the few determinations available of the rare earths are in highly 
potassic igneous rocks. 
45745°—sm 1909——20 
