PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 153 



Diabase. Basalt.] 



Age. Cabotian. 



Remark. Two other sections, made very thin, show still further the volcanic 

 origin of much of this rock. It consists almost entirely of devitritied glass and of 

 rhyolitic fragments, the latter showing their fluidal structure and the former a 

 spherulitic. The grains that remain irresolvable in a thick section are found to consist 

 of a crowded aggregate of microlites which never darken as a mass, and give no 

 separate polarization. Their sombre tint indicates that they consist largely of quartz 

 and of feldspar. Some of the grains were evidently in the condition of granulitic 

 quartzes embraced in a mass of apobsidian. Some still show such partial extinctions, 

 the area of their sections being divided between several orientations. 



It is probable that this rock resulted from a sand of volcanic glass and a few 

 fragments of augite. The parts are hardly sufficiently angular to be referred directly 

 to volcanic explosive action. In many particulars it agrees with the volcanic breccia 

 described by George H. Williams from the Sudbury district of Canada,* but is rather 

 a clastic accumulation than a breccia. N. H. w. 



No. 62. DIABASE (?) (Altered.) 



Duluth. Returns after No. 61; like No. 60, but bedded like No. 61. 

 Ref. Annual Report, ix, page 20. 



Me//. A very fine-grained, compact, reddish-brown rock. 



Mic. Rock much altered and reddened as is usual. Section consists of very 

 small lath-shaped feldspars and a confused mass of magnetite, hematite, chlorite, 

 calcite and quartz. What was the original nature of the rock, aside from the feld- 

 spars, is uncertain. Resembles No. 65. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 63. BASALT (?) ( Amygdaloidal. ) 



Duluth. Overhanging rock which, toward the east, becomes brecciated. 

 Ref. Annual Report, ix, page 20. 



Meg. A dark, much rotted, almost black, aphanitic, earthy rock with numerous 

 small amygdules of chlorite, a few of quartz and less of calcite. Pyrite, in small 

 crystals, is disseminated throughout the rock. 

 No section. 



Age. Cabotian. u. s. o. 



No. 64. BASALT. (Anujf/du/oidal.) 



Duluth. In a heavy bed three feet thick. Overlies No. 65. 

 Ref. Annual Report, ix, pages 20, 22. 



*Sulletin of the Geological Society of America, vol. iii, p. 138; Geological Survey of Canada, vol. v (new series) , Part I. 

 Appendix I, p. 75F, 1893. 



