MlNERALOflY; WITH A CLASSIFICATION OF SILICATES. 75 



kaoliu in composition." Hence it was that when, in the same year the writer found and 

 analyzed a crystalline micaceous kaolin, giving for its composition, silica 4;"). 50-46. 05, 

 alumina 38.37, lime O.Gl, magnesia 0.63, water, 13.90 = 99.56, the analyses of pholerite and 

 of kaoliu were discussed by him, and the conclusion was reached that tlie first ratio men- 

 tioned might represent the composition of both of these substances when free from foreign 

 matters, and consecj^uently that " the mineral in its pure form is no other than a crystal- 

 line kaolin." In 1863, in accordance with this view, kaolin and pholerite were regarded 

 as identical. Of pholerite, it was then said that " it may be regarded as that substance 

 [kaolin] in a crystalline condition. From its foliated or micaceous structure it may be 

 considered as a hydrated mica." ' It should here be added that the writer had in 1S55 an 

 opportunitjr of comparing the crystalline mineral from Canada with the original pholerite, 

 and of discussing the question of the minerals with Guillemin in Paris. 



§ 98. It was not until 1867 that this subject was again taken up, and this time by S. 

 W. Johnson and J. M. Blake," who showed that as regards the composition of kaolin, the 

 ratio of 3 : 3 : 2 was inadmissible, aud that the ratio, 3:4:2, deduced by Forchammer, was 

 the true one. They farther called attention to numerous observations showing the occur- 

 rence of kaolin in a foliated crystalline form, which, according to them, is never wanting 

 in the mineral, and proposed for crystalline kaolin the name of "kaolinite," without, how- 

 ever, alluding to my own j)ublished observations on the subject. This historical sketch 

 will serve to show why the present writer, while recognizing in 1854 aud 1863 the crystal- 

 line character of kaolin, was led, from the uncertainty then prevailing as to the true com- 

 position of these si^bstances, to describe crystallized kaolin under the name of " pholerite," 

 as a hydrous mica. While the title of kaolin to the rank of a hydrous phylloid or per- 

 silicate mica, with the ratios, 3 : 4 : 2, is clear, the name of pholerite must be reserved 

 for a similar species with the ratios, 3:3:2. Intermediate between pholerite and kaolinite 

 in composition is the talcosite of Ulrich, to which he has ascribed the atomic ratios for 

 alumina, silica and water, of 5 : 6 : 1. To these succeed the pyrophyllites, the two for- 

 mulas assigned to which give the same value for P, while the recorded densities indicate 

 for the mineral a high degree of condensation. 



Tribe 15. — Argilloid. 



§ 99. In the last place remains to be noticed the percolloid tribe, to which, from the 

 fact that it includes varions hydrous aluminous silicates known as clays, we have desig- 

 nated Argilloid. While a mineral having the composition assigned to kaolin appears as a 

 phylloid, and while Johnson and Blake, from their microscopic examinations, concluded 

 that such a phylloid species appears in all kaolins, it is not improbable that there ma)'', at 

 the same time, exist a true argilloid or colloidal silicate having the compo.sition assigned 

 to kaolin, and, consequently, less hydrated thnu halloysite. For this reason kaolin is 

 retained in the accompanying table of argilloids. We have noticed in the other suborders 

 the close relations between the ophitoids and pinitoids and their corresponding phylloids. 



See Hunt, Report of Geological Survey of Canada, 1853-50, p. ".80, and further. Geology of Canada, 1863 

 p. 495. 



'' Amor. .Tour. Siàenco, xliii. 351. 



