PETROLOGY IN AMERICA. 463 



Europe. The name applied to it assumes that the border- 

 ing minerals are of secondary origin, due to reactions 

 between the felspar and the ferro-magnesian silicate or 

 iron-ore orain, and several writers have ascribed the effect 

 to dynamic metamorphism ; but on these questions there 

 is by no means a common agreement. In the Canadian 

 anorthosites Adams (1) records a zone of red garnet as 

 frequently occurring between pyroxene or iron-ore grains 

 and felspar. In other cases he notes two zones between 

 olivine and felspar, the first being of a pale rhombic 

 pyroxene, the second of pale green actinolite needles, set 

 perpendicularly and sometimes having parallel interposi- 

 tions of deep green spinel. He finds no evidence of these 

 rims being a secondary effect due to dynamic causes. In 

 the great gabbro mass in Minnesota, as described by 

 Bayley (3, 7), the olivine is often surrounded by a narrow- 

 border of diallage or augite which thus intervenes between 

 it and the felspar. This border seems to be certainly an 

 original growth, for it is sometimes seen to be continuous 

 with a crystal-plate of diallage. Elsewhere there is a 

 fibrous intergrowth of the bordering augite with the 

 contiguous labradorite, and this is found surrounding 

 magnetite as well as olivine. Biotite is also found inter- 

 posed between magnetite and plagioclase, and this Bayley 

 considers due to a reaction, but his reasoning is scarcely 

 convincing. In the anorthosites of the Adirondack^ Kemp 

 states that there are no reaction-rims, although the rocks 

 give evidence of great dynamic disturbance. In the 

 basic gabbros, however, he describes a considerable variety 

 of borders round pyroxene, olivine, and magnetite. Between 

 augite and felspar is interposed a zone of brown hornblende 

 crystals ; between magnetite and felspar a first zone of 

 brown hornblende and a second of garnet. In this latter 

 case there may be additional zones, such as biotite 

 immediately surrounding the magnetite, or clear quartz 

 between the hornblende and the garnet. A curious feature 

 is a parallel or micrographic intergrowth of the garnet with 

 the adjacent felspar. Between olivine and felspar are seen 

 in some cases three successive zones, respectively of granular 



