bascom.] MELAPHYRES AND AUGITE-PORPHYRITES. 73 
The olivinitic porphyrites, or true melaphyres, contain olivine as a 
constituent of the grouudmass as well as in the very plentiful porphy- 
ritical crystals. 
The distribution of the melaphyres is quite similar to that of the 
spilites. The history of the two types since consolidation has been the 
same, and they will be discussed together. 
The vesicular structure is a conspicuous feature of the melaphyres 
and spilites. They range from rocks almost as vesicular as a sponge to 
a compact rock containing* only a few scattered vesicles. These vesicles 
are filled with material furnished by percolating waters, and a solid 
amygdaloid is formed. 
The mineral nature of the amygdules will be described under the 
secondary constituents. The vesicles vary in size from microscopic 
dimensions to 5 centimeters in length and 3 in breadth. They are very 
significant, both of the amount of shearing and of alteration present 
in the rocks which they characterize, and they have undoubtedly been 
a factor in determining the character of both processes (pp. 74-75). 
Secondary structures. — The micropoikilitic structure, as has been noted 
on page 49, is occasionally present. It is found in those melaphyres 
and spilites which have been thoroughly silicified by infiltration. The 
secondary nature of the structure is very plain. The original structure 
(the ophitic) is so well preserved, in spite of the replacement of the 
mineral constituents, that in ordinary light the altered character of 
the rock is scarcely apparent (PI. XIX, a and b). Polarized light at 
once betrays the extent of the alteration. 
Where the schistose character of tlie rock is pronounced in the hand 
specimen, it is also a marked feature of the thin section. The con- 
stituents, which in these cases are for the most part secondary, are 
arranged with their longest axes at right angles to the pressure. 
There has been so complete a recrystallization of the rock as to 
obscure its original character. It has been repeatedly pointed out that 
under conditions of pressure igneous rocks acquire a degree of schis- 
tosity which renders it almost impossible to determine their true char- 
acter and to distinguish authoritatively between foliated traps and 
metamorphosed slates (elastics). In the schists under discussion their 
relation to undoubted porphyrites leaves no room for doubt as to their 
origin, nor is their alteration so extended as has been described in 
other localities. The structure of the South Mountain porphyrites is 
usually far less altered than is the case with the greenstone schists of 
the Menominee and Marquette regions. 
Original constituents. — It is to be expected that these aucient rocks, 
comparatively soft and extremely vesicular, exposed as they have been 
to pressure, percolating waters, and weathering, sbould exhibit altera- 
tion. It is surprising that the alteration has not been so complete as 
to obscure altogether the original structures and constituents. The 
original constituents of the rock— plagioclase feldspar, augite, olivine, 
titaniferous magnetite — are either present in a comparatively fresh con- 
