Gabbroid Rocks of Mmncsota. — Wuichell. 271 
tion differs from the first also, in producing more varied re- 
sults; indeed, while the first ordinarily only gives rise to col- 
orless zeolites and hematite, this produces zeolites of various 
shades, including scolecite, mesolite and pseudomesolite'^ 
tinted by hematite, clinochlore and penninite, and more rare- 
ly magnetite; it may also give rise to calcite, zoisite, and re- 
crystallized fine granular or spherulitic labradorite.f Often 
parts of the original large mass of feldspar still remain wholly 
unaltered, while beside them can be found the neogenetic 
crystals frequently lath-shaped and of varying orientation. 
All about will be found: calcite, zeolites, chlorites, iron oxides, 
and occasional grains of zoisite. Careful search for epidote, 
garnet, quartz, an acid feldspar, and amphiboles, all common 
products of saussuritization, has been in vain. 
This method of alteration shows no distinct stages of de- 
velopment, though it may be divided roughly as follows : 
1. Appearance of fracture planes. This stage is only rare- 
ly observed since it is quickly followed by the next. 
2. Development of zeolites and hematite along the frac- 
ture planes. 
3. Development of other minerals. 
III. There is still an other method of alteration which is 
illustrated in plate X, Fig. 2. While in the methods described 
heretofore the alteration begins on the outside and works in- 
ward along cleavages or fracture lines, in this case the forma- 
tion of new minerals begins within and works o'utward. To 
distinguish more briefly between the two modes of alteration 
which are not confined to labradorite, nor to the feldspars, but 
occur commonly with many minerals, I would propose that 
the former be termed exogenesis, (or exogenetic alteration) 
and the latter endogenesis. Thus the method by which chlor- 
ite is ordinarily formed from hornblende is exogenesis; endo- 
genesis is naturally much more uncommon. It is illustrated 
by the alteration of the feldspar just described, and by the 
formation of gothite at the expense of olivine. In this latter 
case, however, as well as in many others, the two methods of 
*This is a new zeolite which will be described in detail later, 
tit is possible that this may be in part albite, but it has been im- 
possible to demonstrate the presence of the acid plagioclase. 
