DR T. THOMSON on Asbestus, Chlorite, and Talc. 363 



The first remarkable circumstance in this mineral is the total 

 absence of lime. Almost all the varieties, both of amphibole and 

 pyroxene, have been analyzed in my laboratory ; and we have 

 an ample collection of both analyzed by BONSDORF and ROSE ; 

 and not one of either has been met with that did not contain 

 lime as one of its constituents. I think, then, that we are en- 

 titled to consider lime as an essential constituent of both of these 

 species. If so, the specimens of rock-wood which I analyzed, 

 can neither be an amphibole nor a pyroxene. 



The atoms of the bases added together make 38.75, and the 

 atoms of silica are 76. Now, 38.75 x 2 77.5. It would ap- 

 pear from this that rock- wood is composed of bisilicates. 



Farther, the atoms of magnesia and alumina, taken together, 

 amount to 31, which is just equal to 7.75 (the atoms of pro- 

 toxide of iron) X 4. Hence the constituents would seem to be 



4 atoms bisilicate of magnesia with alumina, 

 1 atom bisilicate of iron. 



The water is 1 ^ atom ; but the ^ atom may be considered as 

 owing to the presence of water mechanically lodged in the in- 

 terstices of the fibres. According to this view of the constitu- 

 tion of the mineral, it may be represented by the following sym- 

 bol : 



The mineral which resembles rock-wood most closely in its 

 constituents, is hyalosiderite, discovered by Dr WALCHNER, in an 

 amygdaloid in the Kaiserstuhl, near Sasbach, in Brisgau, and 

 which is crystallized in octahedrons with a rectangular base. 

 But hyalosiderite contains less silica, much more protoxide of 

 iron, and rather more magnesia. Its symbol being 



VOL. XI. PART II. Z Z 



