SECONDARY ENLARGEMENT OF MINERALS. 173 



feldspar in the " labradorite andesite " of the isle of Mull, in which the 

 twins of the core are prolonged as twins in a more acid feldspar. 



An enlargement where the core is unmottled but has a rounded outline 

 is seen at A of figure 1, plate 1. In figure 2, A, there is exhibited a some- 

 what different modification. In this instance, as in some others, the core 

 is mottled and of irregular outline. Two enlargements are indicated by 

 the different extinction angles (Cf. also plate 1, figure 1, B). Second en- 

 largements have been found also at other localities (numbers 3218 2 and 

 3115). In a specimen from near Jug End, Mount Washington (number 

 3139) there seem to be several zones of growth in feldspar.* 



Their Origin. — The fact that the cores of many of the growths which 

 have been described have crystal boundaries makes it extremely improb- 

 able that they can be of detrital origin, though the rocks themselves are 

 undoubtedly clastic. The most probable view of their origin, it seems to 

 me, would regard them as a metamorphic product due to the recrystalli- 

 zation of the detrital grains of the rock, as in the better-known cases of 

 garnet and staurolite.f 



Garnet and its secondary Enlargements. — As already stated, this mineral 

 when in macroscopic crystals is specially characteristic of the central 

 schist bed. It is the common dark red, nearly opaque variety, and 

 occurs in rhombic dodecahedra with truncations by the icosatetrahedron. 

 The crystals vary in size from those that are microscopic to those a centi- 

 meter or two in diameter. Frequently, though not always, they are 

 associated with staurolite. They exhibit the usual characters on micro- 

 scopic examination. They have a decided pink absorption and are some- 

 times compact, though often ragged in appearance from the inclosure of 

 the constituents of the matrix. Minute hair-like interpositions, which 

 are very abundant, are with much probability rutile. In the schist of 

 Johnny's mountain, near Sheffield (number 3114) interesting secondary 

 enlargements of the garnet have been observed. The core or first growth 

 is pink and comparatively free from inclusions, with the exception of the 

 hair-like interpositions above referred to. The rim of secondary enlarge- 

 ment, making from one-half to two-thirds the area of the entire growth, 

 is nearly colorless, and near its junction with the core is filled with an 

 aggregation of cloudy ore material, probably magnetite. Where several 

 crystals have formed in a continuous aggregate, the enlargement incloses 

 the aggregate, just as in other cases it incloses an individual (figure 3, A). 



*Cf. Judd, loe. cit., plate vii, fig. 3. 



fFor the literature of secondary feldspar enlargements in rocks of eruptive origin see : — 

 Haworth, A Contribution to the Archean Geology of Missouri ; Inaug. Dissertation, Johns Hop- 

 kins University; also printed in American (ieologist, May and June, 1888. 

 Judd. Quart. Journ. Geol. Soc. xlv, 1889, pp. 175-18G. 

 Romberg, Neues Jahrbuch fur Mineralogie, etc., Beilage-Band viii, L89?, p. 304, plate \v, fig. 54. 



