PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 463 



Basalt. Amygdaloid. Granite.] 



Remark. The association of these trap sheets with amygdaloidal conglomerates 

 and fine crumbling clastic material, which is wholly wanting, so far as known, from 

 the Animilde, is the guide to their age. 



Prof. J. A. Dodge made an analysis of this red mineral with the following result (Annual Report, xi, 



page 171) : 



Oxygen. Oxygen ratio. 



Silica, 62.73 33.45 14.61 



Alumina, 13.62 6.35 ) 



Oxide of iron, 1.75 .52 \ 



Lime, 5.87 1.67 1 



Magnesia, .65 .26 



Soda, 1.83 .48 ^ 



Potash, .68 .11 J 



Water, 12.25 10.89 4.71 

 The composition seems to bring this mineral under the species mordi'n itf. 



N. H. W. 



No. 635. BASALT. 



i 



Immediately underlies No. 634. Nos. 633, 634 and 635, together, have a thickness of only three or four 

 feet, and are variously affected by proximity to the conglomerate. They seem to have constituted a single thin 

 lava sheet. This rock also has stilbite of the fibrous kind above mentioned. Heulandite coats all seams and joints. 

 li'-f. Annual Report, ix, page 41; Annual Report, x, pages 63, 64. 



Meg. The rock is dense and nearly black, though sparsely vesicular. 



Nlc. Resembles rock No. 632. 



Age. Manitou. 



Remark. There are here three beds of crumbling conglomerate separated by 

 trap sheets which were surface flows, cotemporary with the accumulation of the con- 

 glomerate. N. H. w. 



No. 635A. AMYGDALOID. 



A part of No. 635. 



Ref. Annual Report, x, page 63. 



.!/<//. Coarsely amygdaloidal with all the minerals mentioned under No. 634A, 

 the agates being conspicuous and producing a knobbed exterior surface. 

 No section. 



Age. Manitou. N. H. w. 



No. 636. GRANITE. ( Pegmatitic. ) 



Sec. 1, T. 56-7. Makes a high point with a perpendicular face toward the south. It occupies but a small 

 area at this place, but appears at a number of other places along this part of the shore, and on one or two islands. 

 Compare Nos. 119, 134, 157, 526 and 643. 



Ref. Annual Report, ix, page 40; Annual Report, x, pages 64, 66. 



Ma/. Light-red, very quartzose granite, with no apparent ferro-magnesian 

 minerals, but with small inclusions of what appear to be portions of a basic 

 porphyryte (xenoliths). The coarseness of the grain varies much. 



Mic. The (jiiartz and feldspar have a coarse micro-pegmatitic structure, each 

 mutually surrounding areas of the other, the adjacent portions extinguishing in 

 unison. The feldspar is much obscured by oxide of iron, but it can be observed that 

 extinction is about parallel to the cleavages, indicating orthoclase. 



