80 HORN EXPEDITION — ECONOMIC fiEOLOGY. 



from Pope's and Mount Riddock claims, some of it being of a beautiful ruby tint. 

 The greater portion, however, of this mica was much spotted. No information 

 with reference to the financial aspect of this industry was obtainable, owing to the 

 existence of great rivalry between those interested in the several claims. An 

 examination of the optical properties of this mica shows that the optic axial plane 

 lies across the .symmetrical track or guide-line of the percussion-figure. This fact 

 places it in Tschermak's first order. The angle c:Sl (axis of greatest elasticity) is 

 inappreciable, and the angle between the optic axes is large. Thin cleavage flakes 

 of the dark mica {hiotite) which are of a dark brownish-green colour, give a 

 uniaxial figure, the angle c:a being inappreciable. 



The following explanation is suggested of the observed fact of the occurrence 

 of the greater part of the large mica crystals near the walls of the dyke : — An 

 examination of the hand specimens makes it perfectly clear that the mica was the 

 first mineral to commence the process of crystallising out. As that portion of the 

 granite near the walls, consequent upon the deprivation of some of its heat by 

 contact with the neighbouring rocks, would be the first to cool sufficiently to allow 

 crystallisation to proceed ; it is there that we would Jiave the nuclei of the future 

 crystals formed. The result of this would be the formation of crystals of mica near 

 the walls ; while, judging by the large dimensions of the crystalline constitutents, 

 the bulk of the igneous mass was, and for a consi(leral)le time remained, li(juid. 

 The mica crystals would, therefore, attract to themselves the necessary molecules, 

 which would be enabled to answer to the attracting force as long as the mass was 

 sufficiently fluid. 



The mica crystals would not, however, become distiibuted through the mass, 

 as they would naturally rapidly avail themselves of the molecules that were in 

 close proximity to them, and thus become too large to be influenced by the currents 

 that l)ring within their sphere of attraction the molecules needed for their growth. 

 So an undue proportion of the mica substance would crystallise in close proximity 

 to the walls, leaving the bulk of the rock with less than its due propoi'tion of this 

 minei'al, felspar and (jujirtz greatly predominating towards the centre of the mass. 



C— GARNETS. 



The garnets that were asserted to be rubies, for a short time after their 

 discovery, occur in large quantities in the bed of the Maude and Florence Creeks, 

 which take their rise on the Southern slope of Hart Range, and more or less plen- 

 tifully over the surface of the country drained l)y these cri'cks. The best stones 



