158 TJie American Geologist. September, 1900 
rock throughout the series of changes encloses conglomeratic 
masses of varied nature, which are more or less intact in the 
muscovadyte and less distinct in the gabbro. 
The muscovadyte is always the intermediate phase between 
the greenstones and the gabbro. Sometimes one and some- 
times the other predominates. Nevertheless the gabbro is 
sometimes clearly later than the muscovadyte. 
These three rocks are therefore necessarily allied genetic- 
ally, and since the greenstone is sometimes clearly a clastic 
rock it is the gabbro which has been produced from the green- 
stone and not the reverse ; if the latter had occurred abrupt 
passages would occur between the gabbros and the green- 
stones and the former would be found in a detrital state. 
Prof. N. H. Winchell applies these conclusions to all the 
massive rocks of the region whose present mineralogical com- 
position is accordingly a function of the original mineralogical 
composition of the rocks at whose expense they have been 
formed by the process above outlined. The most acid of the 
latter gave rise to granites, the intermediate to quartz gabbro, 
etc. The transformation of the basic greenstones to gabbros 
is therefore only a particular case of a phenomenon much 
more general. 
The greenstones are primarily of igneous origin, being 
the oldest known rocks of the earth's crust. The great re- 
fusion epoch which produced the gabbro dates from a period 
after or just at the close of the Animikie, in Minnesota. Such 
a refusion naturally produced a molten mass which acted 
under stress like any ordinary eruptive rock, and indeed the 
resultant magma not only produced on cooling in place the 
various gabbros, but also protruded itself as an intrusive be- 
tween the pre-existing rocks in the form of numerous laccolitic 
sills, and dikes in all positions. It further produced lava flow, 
when it reached the surface, and even seems to have produced 
volcanic ash and sinter. 
Over the area of the present gabbro mass the overlying 
rocks have been removed by the varied processes of erosion, 
and by glaciation. If any remnants of the superjacent rocks 
still exist, they are covered by glacial drift, or, at least, have 
not yet been discovered. According to the taxonomy of the 
^Minnesota survey as expressed in final form, the oldest rocks 
