300 GEOLOGY OF THE DENVER BASIN. 



in some cases. Extinction in the zone normal to the albitic twinning plane 

 reaches an observed maximum of more than 35°, which indicates that the 

 Larger tablets are labradorite, or possibly a variety still richer in lime. 



There are mam' smaller tablets <>r staves of plagioclase, with simpler 

 twinning and a smaller extinction angle, and both these varieties are 

 embedded in a feldspathic mass of irregular individuals formed in the last 

 stage of consolidation. This last product is often added to the earlier 

 crystals with coincident orientation, and a part of it is plagioclase, but 

 there is so much of it tree from polysynthetic twinning that in the light 

 of the chemical analysis one is justified in assuming a still larger part to 

 be orthoclase. Some plagioclase tablets are surrounded bv feldspar extin- 

 guishing uniformly parallel to the twinning plane of the tablet. It is 

 this added growth of plagioclase or orthoclase which makes it difficult to 

 distinguish the larger labradorite tablets megascopically. 



Biotite occiii's abundantly and very regularly distributed throughout 

 the rock in little, greenish-brown leaves, which are particularly associated 

 with magnetite and olivine. Some flakes of biotite are included in augite 

 and olivine, but the greater part seems to have developed after those 

 minerals. 



The accessory minerals magnetite and apatite need no special mention. 

 Neither zircon nor titanium minerals have been noticed. 



The structure of this basalt is somewhat unusual, in that the constit- 

 uents formed during the period of consolidation in the dike do not form a 

 groundmass sharply distinguishable from the phenocrysts brought up in 

 the magma from below. The large augite crystals and the olivines are 

 products of the first period of crystallization, and some of the plagioclase 

 crystals also, but the latter grade so gradually into the series of crystals 

 belonging to the second generation that the line between them can not 



lie at all clearly drawn. The study of the other basalts to be described 

 shows that a small number of the plagioclase crystals were in those 

 magmas at eruption. Had the mass of the Valmont dike cooled a little 

 slower it is probable that the resulting rock would have had a nearly 



typical granular structure, and the presence of crystals of two generations 



would have been especiallv hard to detect because of the irregular form 



