ARCH^IAN FORMATIONS. 249 



red quartzite in appearance. The rock is intersected in various direc- 

 tions by veins of quartz. It is also cut into pyramidal masses by 

 smooth, straight fissures, which are usually inclined at an angle of 

 from 60 to 85 to the horizon. In trend these fissures constitute three 

 groups : the first nearly north and south ; the second nearly east and 

 west; and the third northwest and southeast. There are also large 

 irregular fissures, and occasionally points are to be observed from 

 which an unusual number, both of the smooth and the irregular ones 

 seem to radiate. 



The rock is very little affected by weathering, and affords an excel- 

 lent building material, though the form of the blocks is unfavorable, 

 and it is somewhat hard to dress. 



No rock was found in contact with it, but, about half a mile to the 

 southeast, in the line of its trend, the Lower Magnesian limestone ap- 

 pears, into whose horizon the outcrop rises, though it lies chiefly in 

 that of ihe Potsdam sandstone, as shown in the accompanying profile. 



FIG. 17. 



PBOFILE SECTION SHOWING THE RELATIONS ov THE MUKWA GRANITE. 

 1. Outlier of Granite. 2. Potsdam Sandstone. 3. Lower Magnesian Limestone. 



THE BERLIN PORPHYRY. 



At Berlin, thirty miles south of the above, we next find an out- 

 standing Archaean mass, 1 consisting of three large elongated domes 

 arranged en echelon, bearing northeast. The rock is composed essen- 

 tially of small crystals of orthoclase feldspar dissiminated through 

 a peculiar cryptocrystalline base of felsite and quartz, forming a 

 quartz-porphyry. The crystals of feldspar are usually grayish before 

 weathering, becoming reddish afterward. The base in its unweath- 

 ered state very much resembles quartzite, and is of dark grayish cast 

 with a very slight reddish tinge, so modified by its translucency as to 

 give to the whole what may be called a water hue. Yery thin splint- 

 ers may be fused before the blow-pipe with difficulty, forming a 

 transparent glass-like bead. The effect of weathering is marked and 

 peculiar. The color changes to a light reddish, pinkish, or grayish 

 white, and occasionally to a bright red, while the mass becomes opaque 

 and finely granular, and so soft as to be easily cut. There are occa- 

 sionally spots, streaks, or leaves of dark material in the base, which 



1 Comp. Dr. PercivaTs Report of 1856, p. 106. 



