1885. ] Geology and Paleontology. 1089 
two minerals, every grain of olivine is seen to be completely sur- 
rounded by this double zone. 
The zone, next to the olivine, is colorless, or nearly so, but 
often shows a slight pleochroism with reddish andegreenish tints. 
It is formed of small individuals, grown compactly together and 
considerably elongated in a direction at right angles to the sur- 
face of the olivine. When examined with a high power the min- 
eral is seen to possess two sets of imperfect cleavages, and when 
these cross one another at right angles, the direction of extinction 
bisects the right angle. When cut perpendicular to an optic axis, 
it shows the revolving bar of a biaxial crystal. 
The second zone, or that next to the plagioclase, forms a fringe 
about the zone just described and consists of minute needles of 
a light green fibrous mineral arranged at right angles to the sur- 
face of the inner zone and penetrating into the plagioclase, so 
that their examination is attended with considerable difficulty. 
They are, however, seen to be slightly pleochroic with different 
Shades of green and to have an extinction which makes a small 
angle with the length of the fibers. It has exactly the appear- 
ance of actinolite, as that mineral is generally seen in thin sec- 
tions, and that it is really a variety of hornblende is proved by 
the fact that in another specimen of the same rock collected a 
few miles away, in which the zones are still seen in all their per- 
fection, the outer one is no longer fibrous, but shows the cleavage 
and absorption characteristic of hornblende. 
The olivine and the minerals composing the two zones are en- 
tirely differently orientated, and the breadth of the zones does not 
bear any absolutely constant relation to the size of the olivine 
grain as seen inthe slide, since this latter would vary greatly, accord- 
ing to the direction in which the section passed through it. Th 
inner zone, however, is always smaller than the outer one. 
Although the mineral forming the inner zone has the char- 
acters of a pyroxene, no conclusive evidence as to its nature 
could be obtained from the sections of this rock. Similar double 
zones are, however, found about the olivine in rocks from other 
Parts of the area having the individuals of the inner zone better 
developed. In these the pleochroism and rude cleavage are very 
distinct. Ina section of one of these rocks a grain of pyroxene 
exactly resembling the mineral of the inner zone, but not asso- 
Ciated with the olivine, was so cut that it could be proved to be a 
rhombic pyroxene, in that on a basal section known by the cleav- 
ages being at right angles a bisectrix could be seen. It may there- 
fore be said that the inner zone is probably composed ofa rhombic 
obtained. This I propose to attempt, and will make known the 
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