598 /. E. SPURR 



of the phenocrysts, indicates a break in the crystallization, evi- 

 dently resultant from a movement of the solidifying mass. The 

 order of events in this section was, therefore, (i) comparatively 

 slow crystallization of feldspar; (2) flowage, producing a change 

 of conditions; (3) medium rapid cooling, bringing about the 

 uniform moderately fine crystallization of the rest of the rock. 



169 N. Here the line between the abundant phenocrysts and 

 the fine-grained groundmass is, in general, distinct, for although 

 there are transitions between the two they are not so abundant 

 as in rocks like 164 N(a) and 165 N(a). This connotes a 

 shorter period of first crystallization (when the phenocrysts were 

 formed) than does 165 N(a), then a more rapid cooling than 

 165 N(a) to a certain point, then a slower rate of crystallization, 

 permitting the formation of the uniform fine granular structure. 



171 N. The structure of this connotes a long period of slight 

 viscosity, during which the crystals of quartz and feldspar could 

 grow until they touched and sometimes interlocked. The differ- 

 ence in size between these crystals and the grains of the ground- 

 mass or mesostasis which fills the space between them, implies 

 a slight break or change of conditions, after which was again a 

 comparatively slow uniform crystallization of the rest of the 

 rock, producing an even allotriomorphic granular structure. 

 There are then two distinct generations of crystals. The result- 

 ing structure is entirely similar to the ophitic structure of diaba- 

 ses, save that in these siliceous feldspars the forms are not so 

 elongated, and so the structure is not so striking. The struc- 

 ture of this specimen, however, differs from the typical granitic 

 structure in the same way that a diabase differs in structure 

 from a gabbro. A longer period for the first crystallization, 

 reducing the mesostasis to a still smaller percentage, would give 

 the aplitic structure, where the idiomorphic crystals are predom- 

 inant and occupy the greater portion of the section. 



175 N. This is like 171 N, but in general the mesostasis of 

 comparatively small grains is wanting, being replaced by a filling 

 of coarse allotriomorphic quartz. In this case the conditions of 

 crystallization have evidently been gradual throughout. The 



