WEIDMAN — QUARTZ KERATOPHYRE OP BARABOO BLUFFS 49 



is found to some extent as both large and small pheno- 



crysts. 



The phenocrysts vary in size from two-tenths millimeter 

 to four millimeters across. The crystals are more or less 

 altered to sericite, sometimes completely, but at other 

 times only slightly. The alteration takes place very often 

 at the boundary of the feldspar, and forms zones of sericite 

 surrounding the crystal. At other times the fine sericite 

 needles are promiscuously scattered throughout the phen- 

 ocryst. Very often the feldspar is replaced by secondary 

 quartz. At times ' it is only partly replaced, but very 

 often the angular shape which the secondary quartz as- 

 sumes indicates that there has been a complete replace- 

 ment of the feldspar. 



Numerous beautiful examples were observed of bent 

 and broken crystals due to the motion of the magma after 

 the crystallization o£ the phenocrysts (Plate 2, Figs. 1 

 and 2). Broken phenocrysts are not uncommon phe- 

 nomena of effusive rocks. As the viscous streams of lava, 

 including the crystals of feldspar, flowed over the surface, 

 the motion within the mass bent and broke the pheno- 

 crysts. Sometimes they are merely cracked or broken, 

 but very often the broken parts are separated from each 

 other (Plate 2, Fig. 2), allowing a thin stream of ground- 

 mass to flow between the dismembered parts. It very 

 often happens that the crystals of feldspar, both the 

 broken and unbroken ones, have their corners rounded by 

 corrosion. The embayments in the feldspar shown by 

 some of the slides ^ show plainly this resorption. 



Quartz phenocrysts. — This usually abundant mineral as a 

 primary constituent of the acid rocks, occurs very rarely 

 as phenocrysts in the Baraboo quartz keratophyre. In a 

 total of thirty-two slides examined from this region only 

 four were found to contain porphyritic quartz, and these 

 showed only a few crystals, all of which were of small 



1 Section 3084.1 



» Sections 3079 and 3089. 



