NOTES AND EXHIBITS. lOS^' 



eiionsrh, 01' the conditiouF- of coolinor slow onoucrli, to allow of tlic 

 separation of the iron from tho silicates in the foi'ui of iii;ignetite. 

 Then towards the commencement of the newer ei-uption the rock 

 in which the crystals were embedded became liquified, and these 

 crystals entangled in the molten glass were carried upwai'ds to the 

 top of the volcanic orifice. Here part of the lava may have been 

 thrown into the air, and fallen as lapilli, either on to the ground 

 or into water. As no flattening out of the lapilli was observed 

 the latter may have been the case. The crystallites [the trichites 

 and belonites] and pei'haps the microscopic crystals of felspar 

 did not begin to form before the lava began to cool, and thus 

 grouped themselves tangentially around the micro-porphyritic 

 crystals of olivine and augite. The pent-up liquid and gaseous 

 enclosures in the olivine and augite in the lapilli, on the relief of 

 pressure consequent on reaching the earth's surface, probably 

 burst their way out through the sides of the crystals leaving tube- 

 like pores resembling shot-holes, and escaped through the glass to 

 the outer surface of the lapilli, the pores afterwards becoming 

 lined with dendritic dvisty magnetite at the same time that 

 magnetite was separated out in the same form along the cracks 

 in the cooling glassy base. Spherulites, and possibly strings of 

 globulites, formed in the base while it was cooling. The latter in 

 one case distinctly follow a crack traversing olivine augite and 

 felspar crystals as well as the glassy base, which renders this 

 explanation doubtful. These globulites can scarcely be referred 

 to palagonite as they are feebly doubly refracting. The zonal 

 cracks surrounding the olivine may have been due to that mineral 

 cooling more slowly than the surrounding glassy base, and so 

 shrinking slightly after the solidification of the glass, which has been 

 suggested as a reason for the partial devitrification of the latter 

 at its point of contact with the olivine. In connexion with this 

 question it w^ould be interesting to determine the relative conducti- 

 vities of the basalt glass and the olivine. 



(2) Note on the occurrence of Dacite at Moss Vale : — - 

 This rock may be described as a microcystalline quartzose horn- 

 blende andesite. It is composed of felted crystals of triclinic felspar^ 



