442 EVENING DISCOURSES 
in one direction rather than another. Hence we have a heavy compact 
mineral of a glassy texture. 
(6) Pyroxenes and amphiboles —These are composed of strings of tetra- 
hedra, linked side by side by the Fe and Mg octahedra. As is to be expected, 
, a@.Pyroxene Cleavage 
= OI b.Amphibole Cleavage. 
Fic. 5.—The cleavage of pyroxenes and amphiboles. 
they are all fibrous in nature, splitting very easily along the chains but not 
across them. Asbestos is a well-known example of such a mineral. Asbestos 
fibres are most remarkable. One can tie an overhand knot in a fibre and 
pull it tight without breaking it, just as one can with a cotton thread. 
Familiarity lessens our surprise, but it is really extraordinary that a knot 
can be tied in a stone with such ease. This property arises from the very 
