976 THE GEOLOGY OP MINNESOTA. 



[Quartz-porphyry . 



history of the Archean crystalline rocks shows they have experienced, when all their 

 microscopic phenomena are collated and adjusted into a systematic scheme of 

 interpretation. 



Quartz-porphyry. There are two quai'tz-porphyries to which it is desired to 

 direct attention, as they differ considerably, and are of widely different dates, and 

 are perhaps worthy of separation in nomenclature. One is the oldest known acid 

 rock and the other is the youngest acid eruptive of the Keweenawan. These rocks 

 differ not only in age, but in physical and mineral characters. The former supplied 

 many pebbles for the basal conglomerate of the Upper Keewatin. 



The Archean porphyry (Nos. 2229, 2237) is light-colored, weathering whitish. 

 It is mentioned in vol. xiv of this report, where its field appearances are described 

 (pp. 276, 290-293). It exhibits several features of a clastic, at least a sedimentary, 

 or chemico-sedimentary origin. It is not known to act as an intrusive on the older 

 greenstones which lie alongside of it, but it furnished many fragments to the green- 

 stone next younger than itself. (See plate Z, figures 1 and 2, vol. iv. The succession 

 of rocks in order of age is shown by figure 39, page 292.) Its position is between a 

 conglomerate made up exclusively of debris from the Kawishiwin greenstones, and a 

 conglomerate composed largely of debris from itself. 



The place of exact transition from the underlying (or southerly) greenstone 

 conglomerate could not be found, but, throughout an interval of a few feet the 

 porphyry along the contact became fine-grained and green, but spotted with rounded 

 fragments of white and red jaspilitic quartz referable to the older greenstone. The 

 transition belt, however, was seen embraced in a boulder, found in the immediate 

 vicinity, which was so suggestive that a faithful drawing and description were taken 

 on the spot (vol. iv, page 291). From the gradually changing characters, whether 

 the greenstone conglomerate or the porphyry be considered, the contact of these 

 two rocks certainly exhibits features that indicate a sedimentary transition. They 

 approach each other in color and mingle with each other in grain, there being three 

 or four larger alternations of the evident characters of both rocks before the transition 

 is complete. 



This porphyry also appears, in the field, to pass to a graywacke (No. 2232) and 

 its microscopic characters coincide with that hypothesis. In one place quite plain 

 fragmental structures were seen in the body of the rock (No. 2237), these structures 

 being plainly a part of the rock itself and dying out in all directions. 



As to composition, the rock is peculiar for an igneous quartz-porphyry. Its super- 

 ficial extent, north and south, which is across its width, is about 2,000 feet. Its extent 

 east and west is much greater, but is not known. On the supposition that this rock 

 came from a deep-seated magma, its great amount was sufficient to allow a slow 



