402 



pegmatite, together with epidote, which is distributed along cracks, and is 

 evidently, therefore, a secondary mineral. 



Rock. Felspar. Epidote. (l) Remainder. 



Si0 2 73-8 ... 36-22 ... 3-55 34.03 



A1 2 3 12-9 ... 10-32 ... 2-39 ... -19 



Fe a O s 1-0 ... 1-00 



CaO 2-2 ... ... 2-20 



MgO tr. 



K 2 8-4J 



Na 2 -7J 



H -4 -18 -22 



99-4 55-64 9-32 



The composition of the rock in minerals is, therefore, as follows : 



Felspar 55'64 



Quartz 84-03 



Epidote 9-32 



98-99 



Neglecting the epidote the composition of the pegmatite is : 



Felspar 62'05 



Quartz 87'95 



100-00 



If micro-pegmatite is an eutectic compound these figures represent 

 approximately its mineralogical composition. 



The spherulites in many of the acid rocks probably answer in compo- 

 sition to micro-pegmatite. In the last edition of his work KOSENBUSCH 

 suggests that they may be formed of a definite chemical compound micro- 

 felsite in which the ratio of alkalies to alumina is the same as in the 

 felspars, the silica being in excess of that required to form an alkali-felspar. 

 The present writer is inclined to regard these spherulites as due to the 

 simultaneous crystallization of quartz and felspar, and as analogous to the 

 spherulites formed during the consolidation of the eutectic mixture of nitre 

 and nitrate of lead. If this be correct then the spherulites in question 

 are simply special modifications of the pseudo-spherulites and micro- 

 pegmatite. 



SORET'S principle may have important geological applications. Thus, 

 as LA.GORIO points out, it will furnish a plausible explanation of the fact 

 that certain dykes are glassy in their central portions, instead of at their 

 margins as is more frequently the case. Suppose a mass of molten 

 material to be injected into a fissure and to remain stagnant for a consider- 

 able length of time, the mass will be cooled at the margins, and the 

 compounds with which the solution is most nearly saturated will accu- 



(1) The epidote is calculated from TSCHEEMAK'S formulae on the assumption that the lime 

 and iron are wholly present in this mineral. The error involved in this assumption is too slight 

 to seriously affect the result as regards the relative proportion of felspar and quartz. 



(2) Mik. Phy. d. mass, Gesteine, p. 389. 



