Spodumene and its Alterations. 34? 



in length. Various transitions were observed in the disposition 

 of these materials, such as huge pseudomorphs, of which a large 

 core consisted of a coarse aggregate of mica-crystals, with an 

 outer crust one or two inches thick, in some cases of Cymatolite, 

 and in others of grayish-white Quartz. 



IV. 



Muscovite after Spodumene. 



Many pseudomorphs were found in the Chesterfield vein, 

 which consist in large part or entirely of a greenish-jrellow Mus- 

 covite with peculiar greasy lustre. In fact all stages of inter- 

 mixture with Cymatolite were observed, from the almost pure 

 pseudonmrphs in the latter mineral, in which Muscovite occurred 

 only in minute or even microscopic scales, lying mostly parallel 

 to the axis of the crystal — to others, in which the mica was so 

 abundant as to have imparted a yellow or greenish color to the 

 mixture — and at last to micaceous pseudomorphs, perfectly free 

 from Cymatolite, retaining the form and superficial striation of 

 the Spodumene even to the terminations, though the latter are 

 in general more or less flattened in form and distorted. All 

 these varieties of intermixture appear to be rather the results of 

 intercrystallization than of alteration of either one of the pseu- 

 domorphous minerals into the other. The suddenness of the 

 change of conditions which produced the one or other material 

 is often illustrated by prisms, which consist of pure Cymatolite 

 in one part of their length, and of Muscovite generally toward 

 their terminations, with a sharp line of demarcation between 

 the two. 



Another kind of pseuclomorph in Muscovite, less perfect in 

 form, is a variety of those described under III, in which the 

 mica may largely predominate over its associates. In the former 

 kind, the mica tends to occur in continuous enveloping scales, 

 parallel to the axis of the crystal ; while in the latter the mass 

 is always made up of imperfect crystals and flakes of mica, gen- 

 erally from one-half to two inches in diameter, lying confusedly 

 in all planes, and producing a coarse granular structure. The 

 former traverse the smoky quartz in slender and perfect prisms, 

 sometimes six inches to a foot or more in length, and from one 



