220 J. F. KEMP — GABBROS OF LAKE CHAMPLAIN. 



biotite, the succession outside of it then being as above. Zones or 

 " crowns " or " aureoles," similar in alh respects to these except as regards 

 quartz, have been noted by Lacroix in the reference above given and are 

 pictured in the figure there cited. The closeness of the parallel will ap- 

 pear at once to any one who will compare it with figure 2 of this paper, 

 but the replacement of alternate lamellae of feldspar seems not to have 

 been previously noted. These zones are regarded by the writer as sec- 

 ondary, at least in large part. The garnet certainly is, as shown by its 

 relation to the feldspar. The quartz is doubtless residual silica from the 

 excess left in the alteration to garnet of a more acidic mineral — the lab- 

 radorite. Of the brown hornblende and biotite one can st3eak with less 



Figure 3.— Thin Section {number 112) of Oabbro. 



Showing reaction rims of brown hornblende (H), quartz (Q), and garnet (G) between magnetite 

 (Mg) and labradorite (F). The garnet replaces alternate lamellse of the labradorite. Specimen ob- 

 tained from Westport, New York. 



positiveness. The nests of the former without a core of magnetite lend 

 weight to the view that they are secondary. Biotite, forming border scales 

 on magnetite is a very common phenomenon in many rocks. Dr Wads- 

 worth* has figured it as secondary in a Minnesota gabbro, and thinks it 

 due to the reaction between magnetite and feldspar. The writer has been 

 previously inclined to regard it as original.f 



* M. E. Wadsworth: Minn. Geol. Siirv. Bull. 2, 1887, p'ate vi, fig. 1. 



t J. F. Kemp and V. F. Marsters : Trap Dikes in the Lake Ghamplain Valley, etc. Trans. N. Y. 

 Acad. Science, vol. xi, 1891, p. 19. 



