68 THOMAS STERRY HUNT ON A NATURAL SYSTEM IN 



opite, biotite and muscovite, each include chemically vinlike compounds, which should 

 form distinct species. The miiscovites present among- themselves compounds differing 

 like the tourmalines in their atomic ratios, and the micas included under the name of 

 phlog-opite show A^ariations in the ratio of protoxyds to sesqiiioxyds from 2 : 1 to 3 : 2 ; 

 while there are biotites from 1 : 1 to 1 : IJ and 1 : 2. More highly protobasic than any 

 phlogopite is, however, astrophyllite, a titaniferous mica in which the sescj[uioxyd is 

 represented in part by alumina and in part by ferric and zirconic oxyds, and which gives 

 very closely the ratios, 5 : 2 : 10. With biotites having the atomic formula, 1 : 1 : 2, we 

 may note the more basic corundophilite, 1:1:1, near to which, in the ratio of protoxyds 

 and sesquioxyds, come the highly siliceous fluoric lithia-mica, cryophyllite, 8 : 4 : 14, and 

 the basic seybertite and willcoxite. The ratio of protoxyds and sesquioxyds in the latter 

 1 : 2, is that of some biotites, and of the ferric species, annite, near to which is the still 

 more ferriferous lepidomelane, apparently 1:3:4. A like ratio appears in the dense 

 basic chloritoid, 1:3:2, and in the more silicic zinnwaldite, 1 : 3 : fi, followed by the 

 barytic species, oellaoherite, 1:4:6. 



After lepidolite, probably 1 : 4J : 8, and like zinnwaldite, a highly iiuorifiMous mica, 

 remarkable for containing lithia with caesium and rubidium, we come to the muscovites 

 proper, with which the last two species are connected by the fact that their protoxyd 

 bases are alkalies only. The A'ariatious noted in the ratio of these to the sesquioxyds, (in 

 which ferric oxyd replaces a small portion of alumina) are from 1 : to 1 : 9 and 1 : 12, 

 and the ratio of the sum of these to the silica in ditferent analyses is from 1 : 1^ to 1 : IJ. 

 From various muscoA'ites haA'e been deduced the atomic ratios, 1 : G : 9, 1 : 9 : 12 and 

 1 : 12 : 18, with others intermediate, and a careful study would probably show, as in the 

 case of the tourmalines, the existence of a series of muscovites. Near the muscoA'ite, with 

 the ratio first named, must be placed the less siliceous and somewhat calcareous species, 

 margarite, 1:6:4, and farther on, euphyllite, 1:8:9, and cookeite, 1 : 10 : 9. 



§ 89. It will be noted that in this list we have included both hydrous and auh}'- 

 drous species, between which it is impossible to draw a line of demarcation. Phlogopites 

 and biotites are reputed anhydrous, but, as is well known, contain in many cases from 

 two to four hundredths of water, while corundophilite, willcoxite, seybertite, chloritoid, 

 oellachorite, margarite, euphyllite and cookeite, are all more or less hydrous ; the amount 

 of water rising to six hundredths in euphyllite, and to twice that amount in cookeite. 

 Among muscoA'ites, in like manner, water is found in all proportions, up to hydrous 

 species like dainourite and paragonite, w^hich last may be described as a hydrous soda- 

 musrovite. The presence or absence of water in phylloids cannot form the ground of a 

 distinction in classification among phylloids any more than in adamantoids, where we 

 find bertrandite, beryl, euclase, ardennite, epidote and malacone, in all of which water 

 enters as an essential element. 



§ 90. The Avarions minerals Avhich constitute the chloritic group are hydrous magne- 

 sian micas, nearly related to the phlogopites and the biotites. While pyro.sclerite is simply 

 a hydrous phlogopite, with the atomic ratios, apart from the water, of 4 : 2 : 6, penniuite 

 is a less siliceous species, represented by 4 : 2 : 4|. After these come the closely related 

 ripidolite, leuchtenbergite, A'euerite and prochlorite. Venerite is remarkable as an example 

 of a well defined and crystalline chloritic species A'ery near to prochlorite in composition, 



