396 THE GEOLOGY OP MINNESOTA. 



[Laumontite, calcite, etc. 



any instance in outward boundaries. The appearance is as if a rounded fragment, 

 made up of many small fragments of feldspar, had been involved in the rock. That, 

 however, is not a possible explanation, since they are oriented in common, and they 

 must have originated in the rock. They seem to be an instance of feldspar "y/.ulit- 

 I rt i re," to use a term which has been applied by Fouque to incipient quartz. The 

 photographic reproduction of this structure is seen in figure 9, of plate I. 



Two sections. 



Age. Cabotian. 



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f 



o 



FIG. 22. FELDSPAR GRAINS HAVING SIMULTANEOUS EXTINCTION. 



Remark. The identity of orientation in these small feldspar grains points to 

 their being remnants of feldspar crystals which have suffered corrosion by the 

 encroaching siliceous matrix. The poikilitic quartz of the rock appears to be a 

 secondary element and the whole rock may be the result of an extreme alteration 

 of a once more basic one; in which transformation the slow entrance of silica has 

 been the chief feature. The general appearance of the rock itself is not noticeably 



i 



different from the aporhyolytes of the region. These features, and other facts which 

 are brought together in Part III, raise the question of the origin of all the poikilitic 

 quartz of the aporhyolytes, as well as of the aporhyolytes themselves. (Compare No. 

 526.) N. H. w. 



No. 520A. LAUMONTITE, CALCITE, ETC. (Vein matter.} 



From a vein in No. 520. 



Ref. Annual Report, x, page 38. 



Meg. A mass of red laumontite and coarsely crystallized calcite. The specimen 

 seems to represent the width of a vein about three inches. The sides are of calcite 

 and the interior of calcite and laumontite. 



No section. 



Age. Vein in Cabotian rock. u. s. G. 



