50 BULLETIN OF THE UNIVERSITY OP WISCONSIN 



size. All, however, show by their rounded appearance and 

 embayments, the resorption effects of a corroding magma, 

 and thus they indicate that they were once of larger size 

 and probably more numerous. Some 1 of the quartz-pheno- 

 crysts, like those of feldspar, show the effect of a moving 

 magma by being cracked and broken. 



The small number of quartz phenocrysts present would 

 apparently indicate that this rock is a keratophyre rather 

 than a quartz keratophyre. Mineralogically it is a kerato- 

 phyre; but the analyses show it to compare very closely 

 to quartz keratophyre in the amount of silica contained, 

 and the few corroded quartz crystals seem to indicate that 

 the rock was originally a typical quartz keratophyre, though 

 the silica per cent, has been somewhat increased by the 

 secondary quartz present. 



Accessory Minerals. Ilmenite is an abundant accessory 

 constituent occurring in small and large crystals. It is 

 commonly either partially or completely altered to leu- 

 coxene. The alteration to leucoxene along the gliding 

 planes of the ilmenite is well shown in one section. 2 Some- 

 times the ilmenite has gone over to well crystallized sphene, 

 which is commonly mingled with more or less of the im- 

 pure variety. In some cases (Plate 3, Fig. 1) the crystals 

 of ilmenite are partly replaced by secondary quartz, the 

 latter occupying the space of the former, as in the replace- 

 ment of the feldspar crystals. 



Biotite occurs in considerable abundance in a few of the 

 sections. It is crystallized into small blades which are 

 scattered promiscuously throughout the groundmass and in 

 longer blades 3 arranged in sheaves, or in aggregates which 

 seem to be about small cavities in the rock. 



Zircon occurs to a small extent, sometimes being bounded 

 by its usual crystal planes. It is frequently associated with 

 the ilmenite. 



i Section 3073. 

 3 Section 3080. 

 3 Sections 3079 and 3080. 



