STRUCTURAL GEOLOGY. 29 



Origin of the A.rchean granites.] 



magnesian magma, and exists in large amount in the alkaline, is sufficient to warrant 

 a serious doubt as to the actuality of any such transformation. 



It has to be admitted that Hunt refers to the secondary occurrence of orthoclase 

 in diabase in the Keweenawan rocks of the region of lake Superior. This he classes 

 with the occurrence of zeolites, which are naturally produced by the first disintegration 

 of the feldspars of the basic rock. Several occurrences of orthoclase and also of 

 quartz in the Keweenawan diabases have been met with by the survey, but in all 

 cases they have been supposed to be due to the inclusion of portions of the elastics 

 within the diabase, or they are so far removed from the first disintegration that they 

 are easily referable to foreign infiltration. Orthoclastic rock in patches is not an 

 uncommon appearance in the Keweenawan diabases at certain points where the 

 diabases are known to be near the elastics, and the inference is natural that where 

 the clastic rock is not visible, the orthoclastic rock has been carried some distance 

 from its source. The following is taken from the sixteenth report, page 16. The 

 petrography of these occurrences is discussed elsewhere: 



" It is apparently due to the enlargement and multiplying of the reddish felsitic amygdules (?) locally and 

 the specialization of the mineral ingredients into macroscopical crystals that patches of red rock are produced in 

 this greenstone [at Thessalon, Ont.] No. 1161 represents such red rock. Such patches are sometimes four and 

 even eight feet square, scattered capriciously about, visible on the glaciated surface of the dark rock. This red 

 rock consists apparently of quartz and orthoclase in distinct crystals, embracing in their interstices a greenish to 

 black soft substance that, while finely foliated and resembling chlorite, yet does not seem to have served any other 

 purpose than to occupy the vacancies between the other minerals as they assumed their crystalline shapes. 

 Such nodules and veinings, if not such isolated large masses of reddish granulyte in trap rock, are not very 

 uncommon. They occur at Taylor's Falls and at Duluth,and have been described by the writer at several places 

 in northeastern Minnesota. Since the basic eruptives when in their normal state do not embrace the minerals 

 here differentiated within them, it may be presumed that these exceptions are caused by the local and super- 

 ficial mingling of small portions of the siliceous supercrust with the heated basic eruptive. On cooling and 

 weathering the supercharge of siliceous matter is rejected from the mass to fill any convenient veins or amygda- 

 loidal cavities that are within reach. When none such are found these crystals are formed within the greenstone, 

 and are uniformly disseminated in it, causing the well known quartz dioryte and orthoclase gabbro." 



Other isolated occurrences of orthoclase seem to be due to fumarole action, or 

 at least to the introduction of potash by some later infiltration. On Isle Royale, at 

 the Minong mine, it takes the form of adularia in small crystals implanted on copper 

 (itself a filtration product) and on calcite. The total absence of potassium from 

 normal diabase and hence the impossibility of orthoclase taking source from diabase, 

 is the only point insisted on here. Leucite, which is a potash-bearing mineral of 

 later basalts, has not been discovered in the Keweenawan eruptives nor in the 

 Archean greenstones, nor have any potash-bearing zeolites been reported. 



According to the researches of Pumpelly* on the paragenesis of copper and its 

 associates in the lake Superior region, it appears that after the formation of a series of 

 non-alkaline silicates, laumontnite, prehnite and epidote, followed by quartz, there 

 was a concentration of copper in amygdaloidal and other cavities. After the copper 

 and quartz the alkaline silicates appeared, such as analcite (?), apophyllite and ortho- 



* American Journal of Science, vol. ii, September, October, November, 1871. 



