PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 411 



Basalt.] 



aggregations of a chloritic mineral which between crossed nicols shows a fibro- 

 lamellar structure without affording distinct polarization. 



One section. 



Age. An old Cabotian surface flow. N. H. w. 



No. 540. BASALT. ( Coarse. ) 



A mile and a half cast of the mouth of Brule' river (see No. 221). 



Kef. Annual Report, ix, pages 57, 61; Annual Report, x, pages 43, 44, 45, 47; Bulletin ii, page 103. 



Meg. A rather coarse, brownish-gray rock, composed of feldspar, which is 

 sometimes in long striated crystals, and a shining black material which is often in 

 long streams. 



Mir. The section is composed largely of feldspar in tabular, rarely lath-shaped 

 crystals of considerable size, and of less augite in grains which appear to be a little 

 earlier than, or of about the same date as the feldspars. The feldspars show a tendency 

 to an idiomorphic development, and thus give the section a porphyritic aspect. 

 Twinning, both by the albite and pericline laws, is common. A determination of 

 the feldspar by equal extinction angles on either side of the twinning line was 

 unsatisfactory, owing to a lack of sections with the proper orientation. However, 

 h've crystals furnishing bisectrices were found, and these gave very similar results. 

 Two sections having a positive bisectrix exactly perpendicular gave extinction angles 

 of 8 and 9 respectively; while three sections cut almost perpendicular to a negative 

 bisectrix gave extinction angles of 66, 67 and 69 respectively. In the last three 

 the bisectrix was removed only a little from the centre of the field of vision, but was 



* 



well within this field. These results all agree in indicating that the feldspar is 

 andesine. 



Scattered all through the section outside of the large feldspars and augites, and 

 penetrating the cracks, is a dirty yellowish brown, sometimes greenish material, 

 which contains small feldspar microliths and a few small augite grains. Considerable 

 of this material seems to be an alteration product from a former unindividualized 

 base, but much of it still appears to be glassy areas of considerable size remaining 

 perfectly black when rotated under crossed nicols. With this glassy material is 

 some secondary quartz, and apatite and magnetite are common. 



One section. 



Age. Cabotian. 



Remarks. There are two peculiar features in this section. The first is that 

 the feldspar is andesine rather than labradorite, the usual feldspar of the basic 

 Cabotian and Manitou rocks thus far examined. The second is that considerable 

 unindividualized material, which is still glassy, is present. This rock is perhaps 

 a part of the same mass as that represented by Nos. 221 and 223. u. s. G. 



