404 Dr. Mac Gulloch on the Geology of 



accompanied by similar nodules of calcareous spar, often also 

 penetrated by green earth (chlorite baldogee.) This amygdaloid 

 accompanies the^ larger masses of green chalcedony, and seldom 

 extends many inches beyond them. The great masses are either 

 in the form of irregular nodules, reaching from the size of an egg 

 to that of a man's head, or they resemble short veins, of which the 

 opposing sides are flat, and somewhat parallel, and which might 

 perhaps more properly be called angular nodules. Their colour is 

 as various as their other external characters, but the most frequent 

 varieties are comprehended in the following list. 



1. White chalcedony? approaching to quartz, and sometimes to 

 hornstone, in its aspect, fracture, and other quahties. 



2. Pure chalcedony of a more or less milky hue, constituting the 

 chalcedony and the white carnelian of lapidaries. 



3. The first variety of chalcedony, of a brown colour with nar- 

 row parallel stripes of green. 



4. The same, irregularly mottled with green, white, and brown. 



5. The opaque, or first variety of chalcedony, of an uniform dark 

 sap green, with the dull fracture and aspect of wax, and like that 

 substance, translucent on the thin edges. 



6. The translucent variety, similarly coloured, but with a more 

 conchoidal fracture, and of a glassy aspect. 



7. Varieties which in one specimen exhibit a perfect gradation 

 from pure white or colourless chalcedony, to the deepest green. 



All these varieties contain imbedded spherical bodies, varying- 

 from the size of the minutest poppy seed, to that of a mustard seed. 

 These are frequently formed of pyrites, and although they are 

 often entirely decomposed, yet they now and then exhibit the 

 remains of a crystal of pyrites within them, and their external 

 -surfaces retain a ridgy and crystallized aspect, the remains of the 



