ARCH^AN FOBMATIONS. 



251 



Silurian formations, from which it will be seen that it rises to about 

 the base of the Galena limestone. 



Fig. 19. 



North and South Suction through Pine Bluff, 



1. Quartz-Porphyry, Pine BluSE. 2. Lower Magncsian limestone. 3. St. Peters sandBtone. 4. 



Trenton limestone. 5. Galena limestone. 



The Quaetz-Poephtey of Maequette. 



Near Marquette, a little more than twelve miles west of Pine Bluff, 

 very similar quartz-porphyries display themselves in more considera- 

 ble force, constituting a group of jj^ominent hills. A portion of the 

 rock is precisely identical in character with that of Pine Bluff, and 

 the greater mass is but an unimportant variation froni it, but cer- 

 tain portions dejjart from the porphyritic character, and become al- 

 most, or entirely, crypto-crystalline. One variety of this kind very 

 closely resembles the more homogeneous of the red Iluronian quartz- 

 ites, and another is a compact close-textured rock, usually of dark 

 color, but sometimes greenish. I^either of these varieties occupies 

 exclusively any one horizon, but the quartzite-like variety is found in 

 the more southern outcrops, the last mentioned kind immediately 

 north of that, the darker porphyries next, and the coarser, lighter 

 colored ones in the most northerly exposures. 



The iedding is very obscure, but the laminations of certain portions 

 and belts of particular varieties of rock show the strike to be north- 

 eastward. The dip is made out with much less certainty, but ap- 

 pears to be to the northward, and to vary from ] 5° to 45°. 



Though the Berlin porphyry differs from that of Pine Bluff and of 

 Marquette in the absence of glassy feldspar, yet the close Hthologi- 

 cal alliance of the three is very evident, and they doubtless all belong 

 to the same group of the Archaean series. The general strike of 

 these formations, projected westward, encounters several similar out- 

 liers, that are described in Prof. Irving's report, and still further 

 southwest he has found similar qnartz-porphyry overlying the Bara- 

 boo quartzite. There seems to be sufficient reason for regarding the 

 latter as Huronian, so that the porphyries must be regarded as a 

 newer portion of that formation. 



All of these masses present the rounded contour of glaciated sur- 

 faces, and still bear the glacial groovings, and, in some cases even 



