MICROSTRUCTURE OF THE BASIC GABBROS. 



219 



preventing at most points the former from coming in contact with the 

 feldspar which is decomposed.* A few small garnets, marked G, appear. 

 More complex than this is the case illustrated in figure 2. Around a 

 crystal of magnetite, undoubtedly titaniferous, have gathered first a rim 

 of brown hornblende, next a zone of pink garnet, and then the feldspar, 

 with its clear rims and clouded interior; but between the garnet and a 

 hypersthene crystal that is marked Hy, there is a further zone of clear 

 irregular bits of quartz. This series is oftentimes increased by the pres- 

 ence of brown biotite next the grain of ore. A most peculiar change 

 appears in figure 3. The interior grain of magnetite is surrounded as 



Figure 2.— Thin Section (number 231) of Gabbro. 



Showing reaction rims of successive zones of brown hornblende (H), garnet (G), quartz (Q) 

 around titaniferous magnetite (Mg) or hypersthene (Hy) and between them and labradorite (F). 

 The outer portions of tlie labradorite ure clear because of the absence of inclusions. Tlie speci- 

 men was obtained from the hanging wall of Split Rock mine, Westport, New York. 



usual by brown hornblende ; then follows a clear streak of quartz and next 

 the garnet; the last named is not, however, in irregular grains as before, 

 but has worked into the labradorite, replacing its alternate lamellae. This 

 has been noticed on several slides, and gives a very peculiar effect. At 

 times only a nest of brown hornblende fragments remains as the nucleus, 

 the core mineral, if such there were, having been all absorbed. An ad- 

 ditional mineral sometimes appears next the magnetite ^nd is brown 



* Compare Lacroix: Op. cit., p. 245, figure 40. 



