PETROGRAPHY OE CEtENGWATANA SERIES 835 



ment of the primary minerals in such a manner as to present to the un- 

 aided eye an aspect very like the true amygdaloidal structure. This 

 pseudo-structure is not confined to the amygdaloid ; it has invaded the 

 lower and compacter portions of the flows and modified them to a greater 

 or less degree. 



Rock alteration. — Decomposition has progressed further in the amyg- 

 daloidal than in the compact portions. In many instances the amygda- 

 loid exhibits a distinct flowage structure in the parallel arrangement of 

 its constituent minerals, as well as in the elongated form of the amyg- 

 dules. There is a marked mineralogical difference in the several flows 

 of any given succession. 



The secondary minerals. — The vesicles are more or less filled with 

 secondary minerals, laumontite, calcite, quartz, epidote, kaolin, and 

 chlorite being the more common ones. Sometimes the cavities are com- 

 pactly filled from circumference to center, at other times only partially 

 so. It may be that in these latter cases the material has suffered de- 

 composition and removal, thus through a still more complex series of 

 stages attaining existing conditions. Empty cavities are in some in- 

 stances seen — a condition, it is suspected, due to the circumstance that 

 they lie above the datum line of the formation, and are thus subject to 

 the dissolving as well as depositing effects of percolating waters. The 

 walls of such partially filled amygdules are finely textured and bear 

 evidence of the rapid cooling incident to the gaseous filling and expansive 

 force of the vapor of water with which the flow ascended and came into 

 the zone of rock cooling and solidification. The usual phenomena of de- 

 vitrification are strikingly clear in some thin-sections of the amygdaloid. 



Source of the Eruptives 



Nowhere in this region is there seen a suggestion of volcanic craters. 

 The flows beginning to the eastward of Pratt, Wisconsin, and extending 

 west and southward beyond Chengwatana, Minnesota, are traced from 

 river gorge to river gorge more than 100 miles. The contour of the 

 country and its general petrographic habit, the lithology and physical 

 characters of the rocks themselves, and the well known conditions of out- 

 pour and flow as seen in existing volcanoes do not suggest a single spot 

 which may be regarded as the single seat of eruptive activity or the 

 center of a group of volcanic vents. 



Existing volcanoes pour out streams only a few miles long. Localities 

 have been studied which afford a series of vents forming long volcanic 

 lines. Such Geikie points out in Great Britain. In Iceland the craters 

 are in rows so situated that they complement each other in the sum total 



