810 THE JOURNAL OF GEOLOGY: 
ments, then, the alteration would naturally proceed from the 
outside inwards, and ordinarily at an equal rate all around the 
fragment, following its contours. In this way we get zones of 
different mineralogical composition, depending upon degree of 
alteration. This secondary zonal structure can be observed in 
almost any of the sections made from the breccias, but the con- 
centric structure of many of the fragments is especially well 
shown on large weathered exposures. 
The alterations which the fragments have undergone are the 
same which have taken place in the basalt and andesite flows, 
and the prevailing light green color of the fragments has 
resulted from the production of the same light colored second- 
ary minerals. As in the lava flows, some of the denser rocks 
have become almost opaque, and have usually a lighter green 
color than the less altered fragments. An especially well pre- 
served fragment shows the perfectly fresh feldspars lying in a dark 
brown isotropic glassy base. Where this is very thin, globulitic 
devitrification products can be seen, and there also the base is 
no longer isotropic, but very faintly doubly refracting. 
In addition to the rock fragments a few rare ones of large 
plagioclase crystals were found in a tuff, and also in one case a 
fragment of a violet brown augite, the only specimen of fresh 
augite thus far found in any of the volcanics. 
As was stated above, the cement is usually darker in the 
hand specimen than the pieces of rock imbedded init. Under 
the microscope this condition is found to be reversed. The 
fragments as described above remain opaque or nearly so, whereas 
the cement becomes transparent. It is found to be composed 
for the most part of the same secondary minerals as occur in 
the fragments,—epidote, zoisite, fibrous green hornblende, 
chlorite, quartz, feldspar, and a great deal of brownish titanite 
in rounded aggregates and grains, never in crystals. Calcite is 
less common than one would expect, but in some cases it forms 
almost exclusively the cement. Muscovite is rare. Zeolites 
were found to be wanting in the tuffs as well as in the lavas. 
The exact mode of deposit of these basaltic and andesitic 
breccias and tuffs, that is, whether they are true eolian deposits 
