MICROSCOPICAL CHARACTERS OF THE WHIN SILL. 649 
1°36:1:45:1. If we adopt the view of Tschermak and Dolter*, 
and regard the pyroxenes as isomorphous mixtures of double salts 
in which the sesquioxide bases are present in a silicate of the form 
RRSiO,, then we may represent the composition of this mineral 
approximately by the formula— 
14CaFeSi,O, | 
oCaMghi, O, 
6MgMesi, 0, > 
3MgAl, SiO, | 
1MgFe, SiO,. ) 
This formula corresponds to the following percentage compo- 
sition :— 
DIO he te. Sak ay 48°97 
A Oay . ahd itis 4-65 
Heh Ori race: niet h iia 2-41 
HeO ee fs faFto 45, oye 15°23 
Ca@uedsnhl ric tanks 16-06 
Mo Or tase. 8 Katee 12:68 
100:00 
The mineral is remarkable for the large amount of iron, existing 
presumably in a silicate of the Hedenbergite type, and for the 
excess of magnesia over that required to combine with the remain- 
ing lime and the sesquioxide bases. This excess implies the ex- 
istence of the silicate MgSiO,, written in the formula as MgMg%i,0, 
for the sake of symmetry. Whatever theory we adopt as to the 
constitution of the pyroxenes, we seem driven to the conclusion 
that this silicate, known to us as the rhombic mineral, enstatite, 
exists in a monoclinic augite. One point is certain, viz. that the 
substance analyzed did not consist of a visible mixture of rhombic 
_and monoclinic pyroxene. The mineral was uniform in appearance 
and in optical properties. 
The point here raised is one of considerable interest, for two 
reasons —firstly, as bearing on the general question of isomorphism ; 
and, secondly, because in other varieties of the Whin Sill a rhombic 
pyroxene occurs as an accessory constituent, and, in some cases, may 
even be observed intergrown with the monoclinic mineral in such a 
way as to show that its position was governed by general laws of 
erystalline growth. We know that the most powerful object- 
glasses do not enable us to approach the limits of molecular siruc- 
ture, and there is therefore no reason to believe that the micro- 
scopic limit of visibility corresponds with anything definite in the 
nature of crystalline growth. The intergrowth of two minerals 
which can be frequently observed may quite well take place on so 
small a scale as not to be capable of recognition; indeed we have 
in this paper traced micro-pegmatite+ down to a substance in which 
* “Ueber die Constitution der Pyroxengruppe” von C. Délter, Tschermak’s 
Mitth. Neue Folge, Band ii. p. 193. See also numerous other papers by the 
same author in the same journal. 
t Consider also the case of perthite and micro-perthite. 
