32 THE GEOLOGY OP MINNESOTA. 



[The Kekequabic granite. 



able amount the massive rock acquires other names, such as amphibolyte, or 

 eclogyte, and when free quartz disappears they vary to dioryte or syenyte; and as 

 syenyte they still vary to laurvikyte when orthoclase is abundant and soda-bearing; 

 to monzonyte when the feldspars are both alkaline and soda-lime-bearing and the 

 rock contains augite; nordmarkyte when they contain more or less microperthyte 

 of orthoclase and oligoclase, quartz and small quantities of biotite, pyroxene, horn- 

 blende and 8egirine; pulaskyte, which consists of orthoclase in kryptoperthyte forms, 

 hornblende, biotite, diopside, eleolite, sodalite and accessory minerals; shonkinyte 

 when they consist essentially of augite and orthoclase; and yogoyte when orthoclase 

 and augite are in about equal amounts. When free quartz remains there has been a 

 similar multiplication of nomenclature as the other ingredients vary. It is not neces- 

 sary to enumerate these minor variations. They testify to constant fluctuation in 

 the chemical composition, all implying a similar variation in the source from which 

 they are derived. They point to the addition of potash and silica, in constantly 

 varying amounts, to the elements already present in the ferro-magnesian rocks. 



There are microscopical characters which should be noted, indicating a progres- 

 sive crystallization in the fragmentals, leading up to structures that cannot be 

 distinguished from those of massive granites. This is not intended as a general 

 statement simply, for it is so well known that it need not be repeated; but as a fact of 

 special and local observation in an individual rock mass, where the fragmental rock 

 mass manifestly furnishes the material for the intrusive granite, which cuts it. In 

 the field such relations are easily inferred from the study of the major structures* 

 and such transitions have been tentatively assumed in some of the earlier publi- 

 cations. It was not until the rocks collected could be examined in detail that the 

 petrographic transition could be seen to accompany and confirm the structural 

 transitions. But one instance has been fully investigated. It is that of the granite 

 of Kekequabic lake. The details of the petrographic examinations are included in 

 the chapter devoted to the petrographic geology, and the field structures are presented 

 in the chapters descriptive of plates 68 and 80. The evidence may be summarized 

 as follows: 



The Kekequabic granite. There is a small area of a reddish-gray gneissoid granite 

 rising dome-like in the midst of the Keewatin. The clastic strata adjacent consist of a 

 siliceous actinolitic schist, in general terms, but it varies in different ways. The 



hornblendic element becomes coarser and the rock assumes the character of a pecul- 



t 



to have no more clastic microscopical character than the gneisses themselves. It is not improbable that if a block analysis could be 

 made of the whole mass, including both gneisses and evident fragmentals, the resulting composition would not vary strikingly from 

 that of the gneiss of Trembling mountain, or from that of the granite of Carlingford. Indeed, if the whole seven analyses given 

 by Dr. Adams, both slates and gneisses, be averaged, the result is: silica, 64.85; alumina, 15.74; lime, 1.55; magnesia, 2.94; soda, 

 1.39 ; potassa, 3.75, which does not vary much from a normal granite. If to these could be added the lime and quartz of the strata 

 excluded by the selection of sedimentary gneisses only, the result would certainly not fall far from the composition of the normal 

 igneous gneiss of Trembling mountain. 



*Some new features in the Geology of Northeastern Minnesota. American Geologist, xx, July, 1897, pp. 41-51. 



