188 TUE MAEQUETTE lUON-BEARIN(i DISTEICT. 



T\w fnriiiatiou of veins of ferruii-inous nuit(M-i;ils must luive l)een prior 

 to Marquette time, for in the lowest formatiou of the Marcjuette series, as 

 seen on a succeeding page, are found fragments of ferruginous rocks hke 

 the veins in tlic basement Complex. The IFolyokc conglomerate at the 

 l)ase of tlie Huroiiian series in this locality contains very numerous large 

 fragments of \arious ferruginous rocks which are identical with the veins 

 in the green schists helow, and there can l)e no doubt that the ferruginous 

 detritus was derived trom the veins. 



SUMMARY. 



In the preceding pages it is shown that the l)asenient upon which the 

 ^lanpiette sediments Avere deposited, as it exists in the Northern Complex, 

 consists mainly of foliated rocks, including greenstone-schists, gneisses, 

 o-neissoid granites, and syenites, that are cut through and through by intru- 

 sions of acid and basic rocks in the form of dikes and are penetrated by bosses 

 of ])eridotite. All the dikes, except a very few fresh diabases, are older 

 than tlie up})er beds of the Marquette series. They are schistose, and 

 most of them are much altered. The massive diabase dikes were probably 

 formed during Keweenawan time. The peridotite is younger than the 

 Cambrian sandstone and older thaii the greenstone-schists of the Basement 

 Complex. Its age has not lieen determined more accurately. 



The gneissoid granites and syenites differ from one another in com- 

 position, the former consisting essentially of biotite, quartz, orthoclase, 

 plagioclase, and microcline, and the latter of hornblende and the feldspars. 

 Both rocks owe their foliation tii mashing, and both have had developed in 

 them large quantities of new minerals, the most noticeable being microcline, 

 plagioclase, and muscovite. The gneisses differ from the granites simply 

 in the greater perfection of their schistosity and in the greater quantity of 

 new minerals developcid iu them. These acid rocks have the structure 

 of plutonic intrusives. Tlie\' cut tln-ough the greenstone-schists and are 

 intermingleil with them so confusedly that accurate outlining of the areas 

 underlain b\' tiie acid and the basic rocks, respectively, is practically impos- 

 sible. From tlieir structure it is evident that the granites and syenites were 

 intruded into tlie schists when these were at some considerable distance 



