400 THE GEOLOGY OF MINNESOTA. 



[Anorthosyte and diabase. Apobsidian. 



The close resemblance of the groundmass of this rock to the red, so-called 

 felsitic, or feldspathic, substance which encloses the phenocrysts in the quartz- 

 porphyries and rhyolytes of the region, induces us to assume that it has the same 

 origin, and especially as other petrographic characters seem to conspire to unite 

 this rock with those. Hence in the foregoing description it is referred to as a 

 devitrified glass on the assumption that the rock resulted from completely fused 

 material, but with the further supposition that some portion of the magma was 

 interrupted in the process of consolidation, by such unfavorable conditions that 

 the crystallization was not carried to completion. This incompleted differentia- 

 tion, however, is not evinced (it is admitted) so much by the occurrence of actual 

 glass in the rock at the present time, as by the crowded microlitic structure of 

 much of the groundmass, and by the indefinite forms of the crystal grains which 

 did take shape. (Compare No. 520.) 



An analysis of this rock was made by Prof. J. A. Dodge, published in volume i of the Final Report, in the 

 discussion of the building stones, as follows: 



SiO 2 - 71.81 



A1,O 3 12.82 



Fe 2 O, and FeO 6.02 



CaO 2.26 



MgO 0.56 



K 2 O 1.92 



Na 2 O - 2.51 



Total 97.90 



N. H. W. 



No. 527. ANORTHOSYTE AND DIABASE. {Contact.) 



"Sample showing the union and welding of the feldspar mass at Beaver bay, with coarse trap enclosing it. 

 It is not thus welded generally, so far as visible, but is loosened from the trap." 

 Kef. Annual Report, x, pages 40, 41. 



Meg. The hand sample shows a contact between the anorthosyte and the 

 inclosing diabase, which is a fine-grained, almost black rock. The two are closely 

 welded together. 



Mic. The diabase is an olivine diabase with large plates of <m</it?. The anor- 

 thosyte shows that the different grains have been broken, especially along their 

 peripheries, into fine particles, but the thickness of the sections will not permit a 

 careful examination of this point. In the immediate vicinity of the contact the 

 augite and oliviue of the diabase are found in the anorthosyte. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 528. APOBSIDIAN. 



Beaver bay, at the mouth of the creek, and also running back from the creek toward the west. It is in 

 an abrupt and isolated outcrop, but runs below the rock of the promontory. 



Kef. Annual Report, ix, pages 32, 33, 53; Annual Report, x, pages 41, 112,113; same as No. 127, which see. 



