ELIZABETHTOWN AND PORT HENRY QUADRANGLES 57 



some of the ferric iron was in the bisiHcates and it was not entirely 

 combined in the magnetite. The calculation of the pyroxene does 

 not tell us how much is hypersthene and how much is in the mono- 

 clinic variety. Probably some little alumina was in the pyroxene. 

 Yet even with all 'these restrictions the final results in nos. i, 2 

 and 5 are doubtless very near the truth and afford interesting 

 general conclusions. Thus no. i is only 3 per cent higher in silica 

 than no. 5. Yet the light colored minerals are almost 20 per cent 

 greater in the former than in the latter. The excess of alumina 

 in no. I is mainly responsible for this result, since by this we are 

 able to care the better for lime in anorthite. It is striking that 

 silica should fail to satisfy the bases in nos. 3 and 4 on the lines 

 of the ordinary silicates, despite the fact that in these rocks its 

 percentage exceeds that of no, 5, and but slightly yields to nos. i 

 and 2. Mineralogical relationships in rocks, as indeed we have 

 learned forcibly from the quantitative system, depend on other 

 factors than the silica. 



With all their shortcomings these recast analyses are nevertheless 

 presented with the purpose of illustrating again the connection be- 

 tween chemical composition and mineralogical components. 



Unmetamorphosed basaltic dikes 

 The last manifestation of eruptive activity and one which fol- 

 lowed the general metamorphism, took the form of comparatively 

 narrow basaltic dikes. They are quite widely distributed but not 

 specially abundant in this area. They are members of an eruptive 

 series which is widespread throughout the eastern and northern 

 Adirondacks. Approximately 20 individuals or groups of individ- 

 uals have been noted in the Elizabethtown quadrangle, and 16 in 

 the Port Henry. These are undoubtedly but a small fraction of 

 those existing and either concealed or unnoted. Wherever cascades 

 reveal exposures for relatively long distances .or iron mines have 

 opened up the rocks underground the dikes have almost invariably 

 been discovered. The great majority strike northeast. Of 32 

 accurate records, 19 lie between n. 35° e. and n. 70° e. ; 6 are east 

 and west and 4 nearly north and south. Only 3 are northwest. The 

 northeast strike corresponds with the major structural breaks, and 

 the dikes often appear in the broken and jointed rocks where 

 exposed in the beds of brooks. 



The dikes are mostly narrow, from i to 4 feet, but one has been 

 met at 40 feet, and at the other extreme there are some only a 

 few inches. Where the dikes tongue out to a feather edge, they 



