102 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



now filled by quartz, with sometimes a little epidote along the edges; possibly these 

 represent amygdaloidal cavities. 



Mic. The section shows small, crowded, lath-shaped feldspars in a rather sparse 

 groundmass of alteration products. The feldspar is much altered and reddened; 

 many of the crystals show traces of twinning striae. Their exact nature cannot be 

 determined, but from the analysis of the whole rock given below, it is probable that 

 this mineral would fall in the labradorite series. These feldspars are from .25 to .75 

 millimeters in length. The section contains none of the rare porphyritic crystals. 

 The groundmass is clearly secondary and is a confused aggregate of quartz, magne- 

 tite, chlorite, calcite and muscovite. What the original nature of this groundmass 

 was is uncertain. It may have been principally augite (and the rock is a fine-grained 

 diabase), or possibly the finely crystalline groundmass of a porphyryte, or even glassy 

 material. Apatite needles are rather common. There is no indication of what the 

 original ferro-magnesian mineral of the rock was, and there are no areas now filled 

 with alteration products, which might represent porphyritic crystals of pyroxene or 

 hornblende. 



Chemical analysis.* The following analysis of this rock was made by Prof. C. F. Sidener and was pub- 

 lished in the Thirteenth Annual Report, page 100 (chemical series 140), and in Bulletin viii, page xxxiii: 



SiO 2 53.71 



A1 2 O 3 14.96 



Fe,O, 14.45 



FeO 3.65 



CaO 3.35 



MgO 4.59 



K,O 0.56 



Na 2 O 1.40 



H,O 1.60 



98.27 



From the data now available this rock can be regarded as most probably an 

 altered fine-grained diabase. 



Age. Probably Cabotian. u. s. G. 



Remarks. This rock has to be removed from the category of the " red rocks " 

 with which it has been classed in an earlier description.! Its color and the presence 

 of quartz were the characters that led to that classification, but its crystalline feld- 

 spars, which seem to prove original complete fusion, and its rather low percentage 

 of silica, seem to preclude its origination from the acid source which gave rise to the 

 most of those rocks. If, however, the red rocks as a group are referable to the fusion 

 of clastic rocks it is necessary to admit that various degrees of acidity and different 

 stages of crystallization must have been the result. The rock is massive, dense and 

 clean, and has the appearance outwardly of having preserved its characters as well 

 as most of the massive rocks of the region. "Hence it is allowable, still, to suggest 



* Ninth Annual Report, p. 12, 1881. 



i American Association for the Advancement of Science, vol. xxx, p. 163, 1882. 



