38 THE GEOLOGY OF MINNESOTA. 



[Keating, 1833. 



concentric shales." The rock at Patterson's rapids was considered as primi- 

 tive, but was not carefully examined. 



GRANITE IN THE MINNESOTA VALLEY. 



On the afternoon of the 18th of July, Major Long's party first met with 

 unmistakable primitive rock in situ, at a point a few miles below the mouth 

 of the Yellow Medicine river. Of this Keating remarks : 



When descending into the valley from the prairie, with a view to select a suitable spot for 

 our evening's camp, our attention was suddenly called to the new features which it displayed. 

 Hjgh rocks of a rugged aspect arose in an insulated manner in the midst of the widened valley 

 through which the St. Peter winds its way. We spent the rest of the afternoon in examining 

 them, and experienced no little satisfaction in finding them to be primitive rocks in situ. 



The pleasure we experienced sprang not from the mere associations of home, connected with 

 the view of a primitive formation which we had not seen since the first five days of our journey; 

 but it resulted also, in a great measure, from the certainty that we had at last arrived at what we 

 had long been looking for in vain. We had traced those scattered boulders which lay insulated 

 in the prairies from the banks of the Muskingum to this place ; we had seen them gradually 

 increasing in size and number, and presenting fewer signs of attrition as we advanced further on 

 our journey. Two days before, their number, size and features had induced the geologist of the 

 party to predict our speedy approach to the primitive formations, and it was a pleasing confirma- 

 tion of his opinions to find these rocks really in situ, within thirty miles, in a straight line, of the 

 place where he had made this assertion. The character of these rocks was examined with care, 

 and found very curious. It seemed as if four simple minerals, quartz, fieldspar, mica and amphi- 

 bole, had united here to produce almost all the varieties of combination which can arise from the 

 association of two or more of these minerals ; and these combinations were in such immediate 

 contact that the same fragment might, as we viewed one or the other end of it, be referred to 

 different rocks ; while, in some places, granite was seen perfectly well characterized, varying from 

 the fine to the coarse grained ; in others a gneiss, mica slate, greisen (quarts and mica) compact 

 feldspar (weisstein of Werner), sienite, greenstone, and the sienite with the addition of quartz 

 forming the amphibolic granite of D'Aubuisson, were equally well characterized. The only rock 

 composed by the union of two of these principles which we did not observe, but which may perhaps 

 exist there, is the graphic granite (pegmatite, Hauy). These rocks are not very extensive ; the 

 circumference of the largest probably does not exceed one-quarter of a mile ; they rise to about 

 thirty-five feet above the level of the water. Their form is irregular ; their aspect, rugged and 

 barren compared with the fertile bottom of the valley ; their general color is of a dark gray ; they 

 appear to be the summit or crest of primitive rocks which lie beneath this valley, and which pro- 

 trude at this place through the superior strata. As the adjoining prairies are elevated about fifty 

 feet, above the level of the river, these primitive rocks are observable only in the valley ; they 

 doubtless constituted at one time a continuous ridge, but have been divided into insulated masses 

 by the corroding action of the stream, whose very circuitous bed winds between them. They extend 

 upon a distance of about six miles in the direction of the valley. After having examined almost 

 every one of these masses, I feel unwilling to decide, with certainty, which of the primitive 

 combinations predominates, for the passage of the one into the other is more constant and more 

 sudden than in any other primitive formation that has ever come under our notice. Indeed we 

 know of none with which to compare it, except it be that which we observed at a subsequent period 

 of the expedition between lake Winnipeek and the lake of the Woods ; but even there the features 

 were somewhat different, for they were on a larger scale. The passages which we there observed 

 were sometimes to be traced only upon large masses; whereas on the St. Peter it would have been 

 difficult to break off a fragment of a cubic foot in size presenting an uniform character of com- 

 position. It is however probable, as far as our observations extended, that granite is th8 pre- 

 dominating rock. These masses bear very evident signs of a crystalline origin, but the process 





