COUETLAND QUARTZITE AND PORPHYRY DIKE 227 



Both cross-bedding and minor faulting interfere in the measuring of the 

 dip. The most distinct evidence of faulting I discovered in a dike. 



Dikes have not been heretofore reported here, though they were to be 

 expected. The one which I have discovered is on the north side of the 

 railway cut, in the quarry of the New Ulm Stone Company. It appears 

 as a clay seam, a foot wide, perpendicular to the bedding of the quarry 

 rock and crossing the quarry north 25 degrees east. It is exposed some 

 20 feet from the surface downward. The clay is dove-colored, with rhom- 

 bic patches of white, marking it as a rotted or altered feldspar porphyry. 

 The east or hanging wall is slickensided, and the dike is brecciated or 

 crushed near it, so that white patches are drawn out into irregular seams, 

 from which I infer that faulting had taken place parallel to the dike be- 

 fore the porphyry had altered to clay. 



Since this undiscovered dike was at the railroad cut where all geologists 

 have walked, evidently it was not easily recognizable until the quarry 

 worked some distance into the hill. Many more such dikes may exist. 

 The rock of the dike has been less resistant than the quartzite, so that 

 rotting and erosion have made a depression at the surface where it out- 

 crops and the depression is filled with soil and covered by vegetation. 

 Any other dikes would be likewise obscured. The quartzite also graduates 

 from very hard to softer or "sandstone" areas, and these at the surface 

 correspond with erosional depressions. To this the apparent terracing of 

 the hillside is due {10, page 21). The many similar depressions may 

 conceal also a few dikes. 



Both the faulting and the erosion of the quartzite are difficult to rightly 

 interpret without explanation of the phenomena of rock alteration. I 

 take the view that the entire mass of rock now remaining at Eedstone 

 was once all vitrified, and the induration of sandstone to quartzite pre- 

 ceded the intrusion of lava now represented by the dike. Displacement 

 then followed or in part preceded the intrusion. In the geologic ages 

 since that time, the percolation of water down the joints has redissolved 

 the cement, making "sandstone" patches and strata again. This view is 

 not in accord with that expressed by Irving and Van Hise, who say 

 (7, page 34) : 



"At Redstone the transition from argillaceous, reddish sandstone to com- 

 pletely vitrified, brick red to purple quartzite are frequent and abrupt. In 

 places over considerable areas the appearance is as if the rock at higher levels 

 had been vitrified by exposure. But in the railway cutting it is seen that the 

 vitreous quartzites are not restricted to the exposed portions, but are inter- 

 stratified with, and arranged in irregular areas in, an entirely unindurated 

 crumbling sandstone. The peculiarly irregular distribution of the induration 

 and the abrupt transitions from indurated to non-indurated material suggest 

 the possibility of its production by descent along joints and spreading thence 

 through the layers of a silica-bearing solution," 



