BY W. N. BENSON. 715 



of the rock. The action of pneumatolysis is shown in the passage 

 of the plagioclase into scapolite, which is commencing to form in 

 isolated, but optically parallel plates throughout the felspar. 



The limestone, before its alteration, was almost pure, and, in all 

 probability, it had a composition little different from those cited 

 above. 



The types of altered limestones are many and varied, but a few 

 will be described here. 



M.B., 164, is a glistening, white, crystalline rock, consisting 

 chiefly of wollastonite, often twinned, diopside, a little dark green, 

 pleochroic hornblende, and some calcite. Orthoclase occurs inter- 

 stitially, but is very decomposed. There are a few birefringent. 

 reddish-brown garnet-grains, and some irregular grains of a highly 

 birefringent, optically positive, uniaxial, pleochroic mineral, that is 

 probably cassiterite, its refractive index being less than that of 

 rutile. 



There are also white crystalline rocks studded with idiomorphic 

 and granular vesuvianite. Microscopically examined, the vesuvi- 

 anite generally shows a sieve-like structure, containing numerous 

 inclusions of phlogopite and calcite. It is set in a ground-mass 

 composed of calcite, orthoclase, and prehnite, the last showing 

 undulose extinction. The vesuvianite is zoned, and where it exhibits 

 crystal-outline, there are peculiar, comb-like extensions from the 

 crystal-surface into the matrix. The orthoclase is curiously 

 stippled, and the carbonate is scattered through the rock in sharply 

 bounded plates, elongated along (0001), and frequently well termi- 

 nated. It also forms rhombohedra, and irregular grains. A small 

 amount of irregular, brown garnet also is present. There are, in 

 addition, a few small rods, and shorter prisms of colourless diop- 

 side. 



Another type of rock, here, is the prehnite-garnet rock, which con- 

 sists entirely of these two minerals, together with a few, irregular 

 carbonate-grains or crystals, and a little, brown phlogopite. The 

 garnets are in rounded grains and dodecahedral crystals, red-brown 

 in the centre but paler on the periphery. They lie in a matrix of 

 prehnite, with undulose extinction, and rarely showing spherulitic 



