1 



70 



BULLETIN OF THE 



The intrusive rocks belong in general to the basalts^ but arc of course 

 old, and in the majority of cases greatly altered. One probable ande- 

 site as well as intrusive felsites (rhyolites) was discovered. These rocks 

 had never been noted before by the previous observers. One class 

 of the intrusive rocks can be referred to the basalts only with doubt, as 

 the necessary proof of their original composition is thus far wanting, 

 i. e. the actinolite rocks. The evidence is very strong that in the other 

 basic intrusives all the varieties are produced by the alteration of their 

 constituents, and that they were not erupted in their present state. 



It is to be noticed that, while we have found olivine abundantly in the 

 diabases, Dr. Wichmann states that " olivine diabase is not present 

 amongst the rocks from the iron, region of Lake Superior."* 



The " soft hematites" arc doubtless produced by the decomposition of 

 the jasper and its ore, brought about by the fracturing of the rocks by 

 the intrusives and by the secondary action of water, presumably hot, 

 on account of the microscopic characters of the quartz deposited by it. 

 Besides the *' soft hematites" there occur the quartzites and conglom- 

 erates derived from the ore and jasper, as well as the sandstones and 

 schists impregnated by iron, which are sometimes mined to a slight ex- 

 tent. 



We have heretofore seen that the view that the " Huronian " uncon- 

 formably overlies the " Laurentian " has been only supported by the 

 fact that the foliation of the latter did not conform in its dip to the 

 lamination of the former. This proof is of no value unless it can bo 

 shown that both rocks are stratified and in situ. That the latter is not 

 so, we have seen in numerous localities. Heretofore the two systems 

 have not been observed in contact, but recently statements have been 

 published that their junctions have been seen in other regions.t The 

 statement is made that both rocks are stratified, but no proof is adduced 

 to show on what the conclusion is founded, and, although the contacts 

 were said to show beautifully, nothing was published indicating that the 

 kind and manner of the junction was observed. It woiild seem that 

 even here the decision concerning the unconforma})ility was based on 



the foliation only. 



So far as the Marquette district is concerned we have shown very 

 much stronger and more abundant evidence to prove that the " Lauren- 

 tian " granite is younger than the '^Huronian," and an eruptive rock, than 

 has been advanced by Mr. Brooks (the only one who has advanced any- 

 thing called proof) to show that it is older. Further, the inability of the 



* Geol. of Wise, III. 627. 



t Ibid., III. 98, 108, 117. 



