14 ROCK-METAMORPHISM. 



And rarely is a good collection of Norway minerals without 

 specimens exhibiting layers of crystalloidal hornblende alter- 

 nating with others composed of calcite. Specimens of hemi- 

 threne, collected by one of us near Dunglow, in Donegal, have 

 the calcite sharply alternating with layers of small crystalloids 

 of idocrase. Interlamellations of calcite and malacolite (ser- 

 pentine is occasionally present), from Connemara, constitute 

 a variety of what we have called malacolophyte. 



It will thus be seen that a lamellar alternation such as 

 characterizes the " laminated variety of Eozoon " is not an 

 uncommon rock-phenomenon; also that the interlamellation 

 of a mineral carbonate and a mineral silicate, respectively 

 answering to the " skeleton " and ' ' chamber-casts " of this 

 " fossil," is to be observed in rocks every structure of which is 

 entirely of mineral origin. 



Reverting to chrysotile, briefly noticed in the last Chapter, 

 we may be permitted to give a more detailed description of 

 its four stages of modification; for, in doing so, we shall be 

 able to show them in their course of development. 



In the first stage, chrysotile consists of a layer of serpentine 

 penetrated at right angles to its two surfaces by parallel filiform 

 darkish-coloured lines or cuts ; and, though for the most part 

 of uniform length, they are often individually broken into two 

 or more short lengths (Plate IX. fig. 1 a). The cuts are more 

 or less separated, with the intervening space consisting of ser- 

 pentine in its ordinary condition amorphous, subvitreous, and 

 of a green colour. Occasionally this variety assumes a rude, 

 irregular columnar structure, due, we suspect, to the cuts falling 

 into contact, and ranging themselves into divisional planes, 

 which meet one another at no definite angle : the planes are 

 separated by widish structureless interspaces, which constitute 

 the body of the columns (Plate IX. fig. 2 a x ) . Strictly speaking, 

 it cannot be said that chrysotile in either of these states is 

 fibrous rather that we have examples of chrysotile in its 

 incipient stage of development. 



In the second, the cuts are indefinitely increased, so that they 

 cause a layer to appear as if it consisted of a dense mass of 

 fibres (resembling those of asbestos an allomorph of horn- 

 blende). The fibres vary in colour from silver-white to a 

 golden hue, and in lustre from silky to metallic (Plate IX. 



