261 



andesites are, at the time of the eruption, in what may be termed a state 

 of unstable equilibrium. They are liable to be dissolved by the magma 

 and to give rise to other minerals by a rearrangement of the molecular 

 groups. 



An important question arises as to whether they are the products of 

 the consolidation of the magma before eruption or whether they must be 

 regarded as altogether foreign to the magma. The former appears to be 

 the more probable supposition. 



The hornblende of ttie diorites may occur as more or less perfect 

 crystals of the usual form or as grains. In the so-called needle-diorites of 

 GiiMBEL it occurs in long prisms. In colour it is either green or brown. 

 A zonal structure is sometimes present. By alteration it gives rise to 

 chlorite, epidote and finally to carbonates and limonite. 



Aaglte The augite of the andesites may occur either as a porphyritic 

 constituent or as a constituent of the ground-mass. In the former case it 

 usually shows more or less perfect form. It is frequently found in associa- 

 tion with a rhombic pyroxene in which case its crystalline outlines are 

 generally much less perfect than those of the latter mineral. The augite, 

 unlike the hornblende, rarely if ever gives any evidence of having been 

 corroded by the magma. The development of augite at the expense of 

 hornblende has already been referred to. The augite of the diorites is 

 generally pale in colour (malacolite). It is frequently twinned and in some 

 cases shows the diallagic striation. 



Rhombic pyroxene. This is one of the most important constituents of 

 the intermediate group of rocks. It is a ferro-magnesian bisilicate which 

 plays the same role in the intermediate rocks that olivine (a ferro-magnesian 

 monosilicate) does in the basic rocks. In the andesites it occurs in the 

 form of well-developed crystals. The general characters of the minerals 

 belonging to this group have already been referred to and need not there- 

 fore be repeated (see ante, pp. 87 and 163). 



In the diorites (e.y. enstatite-diorites of Penmaenmawr) the crystalline 

 form is somewhat less pronounced than in the andesitos ; the angles being 

 generally somewhat rounded. By ordinary alteration the rhombic 

 pyroxenes pass into green fibrous aggregates. The intensity of the 

 pleochroism varies considerably. The nearly colourless varieties may be 

 regarded as enstatite proper and the strongly pleochroic varieties as 

 hypersthene. It must be remembered, however, that the term enstatite is 

 often used to cover the entire group of rhombic pyroxenes. 



Mica. The dominant mica is a t l>rown biotite with small axial angle. 

 In the andesites it almost invariably occurs in well-formed hexagonal 

 tables. These have sometimes been acted upon by the magma exactly in 

 the same way as the hornblende ; and kernels of unaltered biotite are then 

 seen to be surrounded by a black border of magnetite, with or without 

 augite microlites. 



The biotite of the diorites also occurs in hexagonal tables; the 

 crystalline form is, however, frequently less perfect than in the andesites. 

 Sometimes it occurs in ophitic plates penetrated by lath-shaped felspars 



