2/2 The American Geologist. November, 1900 
alteration often work simultaneously and produce identical re- 
sults,* 
This third method, beginning anywhere within, instead of 
propagating itself along the cleavages or fractures, develops 
outward in all directions, but not with equal speed; it develops 
at once into a rude square or oblong, limited by the cleavages 
/'(ooi) and g^{p\o). This is of course best seen in sections 
which are cut perpendicular to /(ooi) and ^'(010). Hence a 
feldspar altering in this way shows many roughly rectangular, 
square or oblong areas wholly transformed to zeolites with 
some accompanying formation of iron oxides, chiefly hema- 
tite, scattered about irregularly, but with their sides always 
parallel, in the midst of the clear unaltered feldspar. If elon- 
•■gated, the elongation is nearly always parallel to the easy 
cleavage p (001). 
These various decomposition processes considered in an- 
other light and named from end products would be called 
chloritization, calcification, saussuritization, and zeolitization. 
TJie alteration of the augite which is quite common, takes 
place in only a single way. It is often preceded by the appear- 
ance of the diallagic parting which develops first along the 
periphery of the grains and progresses inward. At "the same 
time iron oxides, especially hematite, appear. The diallage 
often alters to chlorite, usually clinochlore.f The augite has 
not been seen by the writer, altering to hornblende, though 
*There is another method of alteration which has not been ob- 
served in the plagioclasytes, but which has been described already. It 
consists in the development of "reaction products," that is, an altera- 
tion of two minerals simultaneously, taking place between the two, 
and usually resulting in the formation of a single mineral containing 
elements derived from both the minerals transformed. Thus biotite 
is formed at the expense of magnetite and plagioclase. While this 
method of alteration begins on the outside and proceeds inward, like 
exogenesis, it differs radically from the latter in that it necessarily 
advances in both directions — away from both minerals, as well as 
towards both minerals. It can, therefore, never advance very far into 
cither mineral, for such an advance would remove it too far from the 
source of certain of the elements necessary to the continuation of its 
growth. Therefore, to emphasize the fact that this alteration must 
always remain between two minerals, the process could appropriatelj- 
be termed mesogenesis. 
tThis transformation of augite to diallage and then to chlorite was 
noted in all stages by Irving: See third annual report, United States 
Geological Survey, p. 102. 
