PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 279 



Basalt. Diabase.] 



The field-note (Annual Report, ix, page 60) indicates that this rock cuts an 

 amygdaloid (No. 249). There are two series of surface flows concerned in the Grand 

 Portage region, and at present it is impossible to separate them geographically. 

 There are also, in the same region, two inseparable series of dikes. N. H. w. 



No. 249. BASALT. (Amygdaloidal. > 



Adjoins No. 218. Third little point east of Deronda bay; probably in N. W. % N. W. J sec. 20, T. 63-6 E. 

 Bef. Annual Report, ix, page 60. 



Meg. A fine-grained, dark greenish to gray, diabasic rock, considerably decayed. 

 Contains amygdules of calcite, quartz, and probably chalcedony. 



Mir. Feldspar microliths in a confused, greenish mass of alteration products, 

 which are largely chlorite, calcite and magnetite. The original nature of this ground- 

 mass cannot be determined, but it is not improbable that it once contained consid- 

 erable gloss. The section contains part of an amygdule of calcite and quartz, in 

 which minerals are a few small grains, with a rather high index of refraction and 

 strong double refraction, which may be epidote. 



One section. 



Age. Manitou ( ?) u. s. G. 



No. 250. BASALT. (Amygdaloidal. ) 



West point of Grand Portage bay; N. W. % N. E. % sec. 16, T. 63-6 E. 

 Ref. Annual Report, ix, pages 60, 61. 



Meg. A fine-grained, brown, diabasic rock, with amygdules of calcite, and with 

 some of quartz and chlorite. 

 No section. 

 Age. Manitou (?) 

 Remark. This is probably the same rock mass as No. 249. u. s. G. 



No. 251. DIABASE (with olivine). 



"Underlying No. 250; an amygdaloidal of a greenish color. These beds (Nos. 250 and 251) dip south at a 

 low angle and do not extend into the bay. They apparently form the coast line between Grand Portage bay and 

 Deronda bay, there being but little outcrop, with a low shore between these places." 



Ref. Annual Report, ix, page 61. 



Meg. A heavy, compact, rather fine-grained diabase. It is dark gray, with a 

 tinge of yellow. The hand sample is not amygdaloidal. 



Mic. The section shows an olivine diabase of medium grain. The olivine and 

 (iityite, especially the former, have been largely replaced by a greenish, yellow serpen- 

 tine like material and magnetite. Possibly, some of this secondary material represents 

 an original glassy residuum. There are some pseudamygdaloidal areas, which are 

 filled mostly with what appears to be thalite, with some calcite and serpentine-like 

 material, and the calcite is also found elsewhere in the rock, especially in the areas 

 once occupied by olivine. One section. u. s. G. 



