dendritic inclusions of an opaque substance, probably magnetite. These 

 (See Fig. 15 (1) ) inclusions lie along the irregular cracks Avhich traverse the 



Fig. 1.5, magnified 100 diameters. Inclusions in olivine of picrite from Rum. 



crystals ; and sometimes are so numerous as to destroy the normal appearance 

 of the olivine and render it perfectly opaque. 



j^ll these inclusions are regarded by Prof. JUDD as of secondary origin, 

 due to a deep-seated metamorphic process for which he has proposed the term 

 " schiller izatioH ." It is not a little interesting to find in minerals of the basic 

 plutonic rocks, so extensively developed on the west coast of Scotland, a set of 

 connected characters, like the above, differentiating them from the correspond- 

 ing minerals of volcanic rocks. 



Schillerized minerals are, however, by no means limited to normal 

 plutonic rocks. They occur in rocks which form an integral portion .of the 

 Hebrideali gueissic system of the extreme north-west of Scotland. 



INTERGROWTHS OF DIFFERENT MINERALS. In ordinary mineral 

 inclusions there is no definite relation between the enclosed and enclosing 

 crystals. The former are taken up mechanically by the latter during the 

 process of crystal- growth. In certain cases, however, we find that two 

 minerals which have crystallised simultaneously give rise to an aggregate the 

 different parts of which have a more or less definite relation to each 

 other. This is the case for instance in the well known graphic granite and in 

 its microscopic equivalent, generally termed micro-pegmatite. Quartz and 

 felspar have here crystallised simultaneously so that the isolated portions 

 of quartz, which frequently exhibit a triangular outline in section, possess a 

 uniform optic orientation, and therefore extinguish simultaneously as though 

 they were parts of one and the same crystal. When the aggregate shows a 

 tendency to the spherical form we have the structure for which Professor 

 HOSENBUSCH has proposed the term pseudo-spherulite, and which has been 

 also designated " centric structure." Micro-pegmatitic and centric structures 

 are especially characteristic of the granites of St. David's, (Dirnetian) and of 

 the Mourne Mounts, Ireland ; of the felsite (granophyre) of Carrock Fell ; of 

 the augite- granites of the Cheviots ; of the granites with augite and horn- 

 blende (perhaps secondary) of Oharnwood Forest ; of the contemporaneous 

 veins and concretionery (?) patches in the bronzite-diabase of Penmaenmawr, 

 and many other British rocks. 



(1) Figs. 1*2, 13, 14 and 15 are copied from the plates illustrating Prof. JUDU'S paper. 



