168 WILLIAM J. MILLER 
Olivine was noted in but one case (No. 1) and this in the only 
rock from which hornblende is missing. 
Titanite in a few small grains was noted in No. 11. 
Reaction or corrosion rims.—Reaction or corrosion rims, which 
are well known in many basic rocks, are exhibited in a truly remark- 
able manner in the North Creek region gabbros. In the examples 
most often described, the core is olivine, but in the gabbros here 
considered the writer has observed cores of olivine, hypersthene, 
magnetite, augite, and diallage with from one to five distinct, 
successive rims surrounding the cores. Professor Kemp has 
described’ and figured a number of interesting examples of reaction 
tims observed in certain gabbros of the eastern Adirondacks. 
The following nine types of reaction rims comprise most of 
those noted by the writer in the North Creek gabbros: 
1. Ilmenite surrounded by hornblende. 
2. Diallage surrounded by hornblende. 
3. Augite surrounded by hornblende. 
4. Hypersthene surrounded by garnet. 
5. Hypersthene surrounded by successive zones of biotite and 
hornblende. 
6. Olivine with successive zones of hypersthene, hornblende, 
and garnet. 
7. Olivine with successive zones of hypersthene, biotite, and 
garnet. 
8. Hypersthene with successive zones of biotite, feldspar, and 
garnet. 
g. Ilmenite with successive zones of biotite, hornblende, garnet, 
and biotite. 
In nearly all cases the material immediately inclosing the 
rims is feldspar which, in a sense, adds another zone to each of 
the above. No. 6 is like one of those described by Kemp. No. 9 
is a remarkable example and, because of its additional outer rim 
of biotite, is even more interesting than a case described by 
Lacroix.? Some of the others may be new examples. The material 
of each rim appears to be highly granulated or at least made up of 
numerous small grains. It seems certain that where hypersthene 
t Geol. Soc. Amer. Bull., V (1894), 218-21. 
2 Bull. Soc. Min. Fr., XII (1889), 232. 
