24 



University of California Publications. [Geology 



variation of extinction of 16° from center to periphery. The 

 extinction of labradorite was obtained for the peripheral zones. 

 Squares of magnetite 0.12 mm. are frequent, and small and large 

 sized crystals of apatite are abundant. Some flow structure is 

 exhibited for the earlier products of crystallization. The ground- 

 mass remains equally luminous between crossed nicols when the 

 stage is rotated; and with a high power and intense light there 

 seems to be no glass present; and apparently it is wholly felds- 

 pathic. The larger phenocrysts of feldspar exhibit the phe- 

 nomena of resorption and subsequent growth, and their outline 

 is less regular than the smaller sized ones. Calcite, as an altera- 

 tion product, pervades both the groundmass and the phenocrysts, 

 and chlorite occurs only in 142. The hornblende of No. 146 

 shows a marginal lighter color and a breaking down into a fibrous 

 or leafy aggregate which is notably free from chlorite. The horn- 

 blende is often fractured and dislocated. Apatite occurs in stout 

 prisms, and in some instances without idiomorphic outline. Mag- 

 netite, which is in grains rather too large to be considered sec- 

 ondary in this rock, takes form against the apatite irregularly, 

 with a tendency to inclose it. This indicates that the magnetite 

 and apatite had crystallized contemporaneously, but that the 

 magnetite had continued longer to crystallize. Some of the apa- 

 tite shows embayments and inlets suggestive of corrosion, and 

 in these are calcite. It is also somewhat fractured, and it is per- 

 haps in the groundmass that the record comparable to that 

 already found in the granite, of the general dynamic effects 

 which are the result of the deformation of the region, are found. 

 The flowage of the molten rock, which more or less fractured the 

 phenocrysts, makes it evident that, in studying the dynamic 

 effects as revealed in the rock, any disturbance that affe'cted the 

 groundmass after it crystallized should be distinguished from 

 that which had affected the phenocrysts during their movement. 

 In the case of the phenocrysts the effects due to movement in the 

 magma are, in the present rock, greater than those due to defor- 

 mation after the rock had crystallized: hence it is in the ground- 

 mass that the record is clear. The effect is apparent through 

 the groundmass, but it does not show any indications of dynamic 

 mineralization. 



