OF THE NOETH-EAST OF FIFK. ■ 43^ 



the enstatite-crystals form aggregates, and in these the mineral is 

 always found undergoing decomposition into viridite, which in turn 

 gives rise to the formation of secondary magnetite and other iron- 

 oxides. 



Fig. 3. "Viridite" pseudoniorph after enstatite, from the rock of Northfield 

 Quarry, as seen magnified 400 diameters. The enstatite has first been 

 converted into bastite (see fig. 1), and eventually into a pale green 

 amorphous substance in which only a faint fibrous structure can be 

 detected. 



Fig. 4. "Viridite" produced by the alteration of the ferro-magnesian silicates 

 in the rock of Causewayhead Quarry, filling up accidental cracks in 

 the rock and forming veins in it. The section is represented as seen 

 with a magnifying-power of 250 diameters. 



Fig. 5. Portion of the ground-mass of the Northfield rock in which "viridite" 

 has been extensively developed (as seen magnified 400 diameters). The 

 viridite has been converted into a mass of spherulites, each of which 

 gives a beautiful black cross when viewed between crossed Nicols, In 

 the midst of tins patch of green minerals circular spots of colourless 

 substance make their appearance, and as these become distinctly de- 

 veloped and increase in size, centres of "opacite" (secondary mag- 

 netite) make their appearance. These white zones in turn assmne the 

 characters of spherulites, and in many places the " opacite " is seen 

 passing into ferrite (hydrated ferric oxides). 



Fig. 6. Development of secondary products in globiform aggregations, along 

 the perlitic cracks and aroimd the microlites of the first consolidation, 

 in the dacite glass of Tay Bridge End, Fife. (Compare Vogel- 

 sang, ' Die Krystalliten,' p. 145, Taf. xv. figs. 4 and 5.) In the 

 clear unaltered glass may be seen examples of tlie star-like aggrega- 

 tions of trichites of this rock. The section is represented as seen 

 magnified 250 diameters. 



Fig. 7. Dacite of Tay Bridge End, Fife, as seen magnified 35 diameters. The 

 rock consists ofufclsitic base, in which great alteration of the original 

 structure has taken place, the development of the hydrated ferric 

 oxides giving it a reddish-brown colour. Traces of the original flow- 

 structure can, however, be detected. Through this base microlites, 

 probably of orthoclase and belonging to the second consolidation, 

 are scattered in profusion. In addition we find large porphyritic 

 crystals, often rounded and broken, of a plagioclase felspar and a few 

 bent and frayed plates of biotite. 



Fig. 8. Dacite glass (vitreous form of last rock) from Tay Bridge End, Fife, as 

 seen magnified 35 diameters. This glass is, in many places, almost 

 perfectly fresh and unaltered. It contains a number of trichites, 

 some of which are grouped in star-like aggregates (see fig. 6) with 

 small felspar-microlites, the whole exhibiting by their arrangeiuent a 

 marked flow-structure. Most conspicuous, however, is the striking 

 doubly perlitic structure of the rock. In addition to the straight 

 cracks, we find a large series of curved ones, which enclose a second 

 and smaller series. Along these cracks decomposition to a greater or 

 less extent is found set up in different parts of the mass (see fig. 6). 

 None of the large porphyritic crystals of plagioclase felspar or biotite 

 are seen in the part of the section figured. 



Discussion. 



Dr. Evans, who occupied the Chair during the reading of this 

 paper, observed that Prof. Judd had added greatly to the value 

 of Mr. Durham's paper. The passage from the original form of 

 volcanic products into the various minerals described was of great 

 interest. 



Mr. RuTLEY doubted if a distinction were needful between ande- 



