C. H. Smyth, Jr. — Metamorphism of a Gdbbro. 277 



amount of hornblende seems to bear a somewhat peculiar rela-. 

 tion to the structure of the rock. Always fairly abundant, 

 even in the most massive specimens, it increases as the rock 

 takes on a cataclastic structure, but as this passes into a dis- 

 tinctly gneissoid structure, the relative amount of hornblende, 

 as compared with pyroxene, decreases. 



The other constituents of the normal gabbro, such as mag- 

 netite surrounded by secondary biotite, titanite, and rare 

 tourmaline, are of minor importance, and demand no special 

 description. 



Of the modifications of the gabbro, only the gneissoid 

 variety requires consideration. While the passage from gabbro 

 to gneiss is gradual, as seen on a large scale in the field, it 

 appears quite abrupt in the microscopic sections, the original 

 structure breaking down rapidly and giving place to the second- 

 ary structure. Two types of this secondary structure appear, 

 which, though closely connected, present much difference in 

 thin sections. In the first type, the rock as a whole retains its 

 original structure, the outlines of the different minerals being 

 slightly changed, but all of the constituents are, at the same 

 time, reduced to fine grains, giving a cataclastic structure. As 

 the crushing becomes more intense, the mineral outlines 

 change, and a more or less pronounced parallel structure 

 results. In this stage the rock is a massive gneiss made up of 

 very fine grains of pyroxene, hornblende, feldspar and scapo- 

 lite. The grains of the two dark and of the two light min- 

 erals are aggregated in large masses, so that the rock appears 

 much coarser than it really is. 



In the other type of secondary structure, the rock is a well 

 defined gneiss. It is apparently much finer grained than the 

 previous variety, but in reality is coarser, the individual grains 

 being many times larger than in ' the cataclastic rock, but not 

 aggregated in large patches. The pyroxene and the horn- 

 blende do, however, form fairly large masses, but quite differ- 

 ent from those described above, being elongated in the direc- 

 tion of the foliation, and often passing around and enclosing 

 the feldspar grains. This rock is composed of feldspar, with 

 pyroxene or hornblende, or, more generally, both. Between 

 this phase of the rock and that with cataclastic structure inter- 

 mediate phases occur, showing that the development of cata- 

 clastic structure is the first step in the process of converting 

 the gabbro into gneiss. The changes w T hich have taken place 

 in the completed process become apparent upon comparing the 

 minerals of the gneiss with those of the original rock. 



The feldspar of the gneiss is in equidimensional grains, 

 instead of lath-shaped, and is quite free from the dusty inclu- 

 sions of the original gabbro feldspar, being water-clear, except 

 when kaolinized. While, in the gabbro, the feldspar is always 



