64 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Almost always, however, the rock has undergone changes to a greater 
or less degree. The.commonly resulting alteration-products are, as Pumpelly 
has shown, from the augite a green and greenish-brown chloritic substance, 
often with red-stained cracks; and from the feldspar a true chlorite, ante- 
cedent to which has been in many cases a change to prehnite. The follow- 
ing quotations are from Pumpelly’s account of these changes.’ The augite 
has changed first, unless there has been a little residuary base present. 
Generally, in any thin section of the lower portion of a bed, a considerable pro- 
portion of the pyroxene is fresh, either throughout whole individuals or in parts of 
these. 
In thin sections, by ordinary transmitted light, the pseudomorphous product is 
translucent, faintly light-green, with a tinge of brown. Between crossed nicols, in its 
most characteristic form, it shows irregular lamellar aggregate polarization. Itis very 
soft under the needle, and is traversed by red-stained cracks, corresponding to the 
irregular fissures in the parent pyroxene, and it is by these, together with the structure 
as seen in polarized light, that it is generally best distinguished from the product after 
residuary magma-base. 
The mineral forming these pseudomorphs is very probably the result of a process 
which has removed lime and some iron, magnesia, and silica from the pyroxene and 
brought in water, and it is, probably, poor in alumina. 
The plagioclase is generally the last constituent that has been altered to any 
great extent. The usual product is chloritic. It is very usual to observe very minute 
particles of a green, apparently structureless substance, suspended in the interior of 
the feldspar in such a manner as to render the supposition quite possible that they are 
due to an alteration of inclosed particles of hyaline base. But an actual pseudomor- 
phism of a chlorite after plagioclase is observable on a large scale. In the first stages 
small, tuft-shaped particles, consisting of laminz, or fibers, radiating from a point, 
occur scattered through the interior of the feldspars, and these may wholly occupy a 
considerable portion of a crystal, while the rest still shows twin striation in polarized 
light. In the finished state no trace of the feldspar is visible except the outlines. The 
pseudomorph then shows an aggregate polarization, due to a confusedly-felted mass of 
minute chlorite tufts. The substance is poorly characterized. * * * * 
There is another occurrence of chlorite in which the progress of growth is from 
within outward. Throughout the pseudo-amygdaloid occur grains of chloritie sub- 
stance, which, in places, reach a diameter of one-fourth to two-thirds inch, with often 
more or less irregular outlines, often nearly round or oval. These consist of a dark- 
green mineral, with H=2.5, which fuses B. B. at 3—3.5 to a black magnetic slag. In 
different beds its texture under the hand-glass varies from amorphous to finely 
scaly. In thin sections, in polarized light, the substance often resembles closely that 
in the pseudomorphs after plagioclase, except that it shows evident growth from within 
outward. There is no defined wall, as of a pre-existing cavity, but the chlorite often 
1R. Pumpelly, Metasomatic Development, pp. 270-272. 
