276 C. H. Smyth, Jr. — Metamorphism of a Gabbro. 



Thin sections always show some alteration in the minerals, 

 and this is usually considerable. Where there is an approxi- 

 mation to the original character of the rock, the minerals are 

 plagioclase, pyroxene, magnetite, and titanite as primary con- 

 stituents, together with a variety of secondary products. Of 

 the two chief constituents, the feldspar is the older, tending to. 

 assume a lath shape. It is well twinned, and the lamellse extin- 

 guish at about 25° as a maximum, indicating labradorite. Twin- 

 ning on the pericline law is often present, in addition to that on 

 the albite law. The feldspar is filled with the line dust-like 

 inclusions so common in the feldspar of gabbros. It is always 

 largely altered, and is often entirely replaced by secondary 

 products. The most conspicuous of these is scapolite. This 

 mineral generally begins to form on the margin of the feld- 

 spar, gradually extending inward until the latter is wholly 

 replaced. Ou account of this mode of growth, the clouded 

 feldspar individuals are ofteu separated from the pyroxene by a 

 clear zone of scapolite. The mineral is readily recognized by 

 its parallel extinction, strong double refraction, uniaxial figure 

 in convergent light, and negative optical character. 



Where the rock shows no effects of crushing, the scapolite is 

 in rather large plates, but in cases where cataclastic structure 

 is present, it is, like the other minerals, reduced to small grains, 

 indicating that it was formed prior to the crushing. 



The second product of change in the feldspar is a fine- 

 grained semi-opaque aggregate. This is possibly saussuritic in 

 its nature, but the opacity and extreme fineness combine to 

 render a determination of its constituents impracticable. This 

 method of alteration becomes most conspicuous in the reddish, 

 earthy portions of the rock referred to above. 



The pyroxene is colorless, and often contains abundant fine, 

 dark inclusions. The maximum extinction angle observed is 

 44°. It forms large plates, filling the spaces between the 

 feldspars. It is always more or less changed into deep- 

 green hornblende, usually in irregularly oriented scales. This 

 change begins on the exterior of the pyroxene and extends 

 toward the center. The inward growth may be very regular, 

 showing a central area of nearly pure pyroxene, surrounded 

 by a well defined zone of hornblende. Or, on the other hand, 

 the hornblende may penetrate the pyroxene very irregularly, 

 yielding a network of the former mineral inclosing residual 

 grains of the latter. Usually, a considerable part of the horn- 

 blende is oriented parallel with the pyroxene, but there is 

 always an abundance of scales irregularly arranged, and these 

 are not uncommonly in excess. There is invariably some of 

 the hornblende present and it increases at the expense of the 

 pyroxene, up to almost entire replacement of the latter. The 



