278 0. H. Smyth, Jr. — Meta?norphism of a Gabbro. 



well twinned, in the gneiss there are abundant grains, often 

 more than half of the total feldspar, showing no twinning, 

 and, in general, when twin lamellae are present, they are much 

 broader and less frequent than in the original roek. While 

 some of the grains show extinction angles almost as large as in 

 the gabbro, in most it is from 5° to 15° lower. Adjacent feld- 

 spar grains are distinct individuals and evidently not mere 

 fragments of larger individuals. 



For these facts, but one explanation seems acceptable : the 

 feldspar of the original rock has been entirely recrystallized, 

 forming again plagioclase, but most of it more acid, while the 

 abundance of untwinned grains points, almost with certainty, 

 to the production of much orthoclase. Moreover, while in the 

 cataclastic variety crushed scapolite is abundant, in the pres- 

 ent type this mineral is generally absent. Evidently, meta- 

 morphism has changed it back into feldspar, from which it was 

 originally derived. 



The pyroxene of the gneiss differs from that of the gabbro 

 in being free from the black inclusions, and in having a deep 

 green color, with faint pleochroism. A difference in form is 

 necessitated by the structure of the rock, and, as above stated, 

 it often builds elongated grains, united to irregular spindles, 

 commonly with parallel orientation over a considerable area. 

 The pyroxene is moulded upon and often encloses the 

 feldspar. 



Here again the change in character and in form can be 

 explained only by recrystallization. 



The hornblende occurs precisely as does the pyroxene; has 

 lost its scaly form, is often bounded by its cleavage planes, 

 and sometimes by crystal faces, and is rather browner than in 

 the gabbro, with more intense pleochroism. It appears to be 

 the result of recrystallization of the scaly secondary horn- 

 blende of the gabbro, this having entirely disappeared. 



The gneiss further contains many grains of titanite, the 

 amount of this mineral being much greater than in the un- 

 changed gabbro, where it is a scanty accessory. Two sections 

 show a few grains of epidote. These minerals doubtless 

 formed from materials separated out in the process of recrys- 

 tallization. 



The foregoing facts lead to the conclusion that the extreme 

 effect of metamorphism on the gabbro has been to produce 

 complete recrystallization, yielding a granulitic structure. But 

 in spite of this recrystallization, the mineralogical composition 

 is not greatly changed — feldspar yielding feldspar, pyroxene 

 yielding pyroxene, and hornblende, itself secondary after 

 pyroxene, yielding again hornblende, and some pyroxene. Yet 

 in each case the recrystallization has produced not only changes 

 in external form of the minerals, but also modification of 

 internal structure and of composition. 



