TRE EASTERN AREA. 381 



surface porphyritic flow in Sees. 29 aiul 3<i. T. 17 X., R. 43 W., ^lirhifraii, 

 and by the large eruptive exposures in Sees. 14, lo, and IH, T. 47 N., R. 

 44 W., Michigan, a.s well as ]»y the great mass of greenstf ine-c< mghmierate 

 itself. This volcanic activity is an exceptional thing in the Penokee sue 

 cession, and in it is believed t<> lie the key which explains the verv excep- 

 tional characters of the series in T 47 X., H. 43 and 44 W.. Michigan. 



TABtLATKiN OF PETROGKAPHKAL OIJSEK VATIONS.' 



1. Chloritic slate. Specimen 12689 (slide .5410i, from 16.5.". N., 60 W.. Sec 16, T. 

 47 N., R. 44 W., Michigan. 



The rock is a dark greeu, fine grained, schistose one, containing large vaguely 

 outlined areas which seem to be of a conglomeratic character. 



The thin section .seems to be that of a fine grained pori)hyritc which lias once 

 liad a glassy background. The background ha« almost wliolly devitriticd. At one 

 side of the .section it con.sists of a turbid gray amorphous aggrejiatc Tliis ;rra<lnally 

 changes into green nonpolarizinsr chlorite in passing towards the other side of the 

 section. TLe background contains many minnte tabular plagioclases, gray areas of 

 leucoxene (wliich here and tliere include paitides "f unaltered inenaccanite;, and 

 some finely crystalline quartz. Tlic .section cftutains also a fewjK>rphyiitic (-rystals of 

 plagiocla.se and intersecting veins of quartz. 



2. Greenstone-conglomerates. Specimens 9.360 'slide .mii'i. 42.5 N., 12.50 W.; 

 9369 (.slide .303.5). 36.5 >'.. 132.5 W., Se«-. 1.5, T. 47 N., R. 44 W.. Michigan. 



The matrix of the conglomerate is a mottled green color, fine grained, and quite 

 Diassive, althou^^h traces of a sehis!ose structure are shown. The larger jiart of the 

 contained ]»ebbles are <»f a pale green color, aphanitic. massive, and often with dly 

 «Iefine<l outlines. A few jiebble.s are of a dark brown coloi-. well rounded and clearly 

 defined. 



In thin .seition the fragments are seen to l>e from basic eruptives. A few of 

 them are well rounded, but they are mostly extraordinarily irregular and sharply 

 angular in form. In places they are .so clo>ely packed together that their outlines are 

 ■with difficulty determined. As a result of these peculiarities the .sections present a 

 striking appearance, as if made nji of a jiileof various kinds of particles having the 

 greatest jiossible irregularities of outline. The majority of the fragments have a 

 gray partially devitrified glas<y back^rround. in which are set small tabnlar crystals 

 of i)lagioclase: in others there seems to have been an amor]>hous background, but it 

 is now wholly devitrified, beiufi comptjsed of chlorite, gray feebly ]K>larizin;; material 

 and somewhat brilliantly polarizing minnte nee<lies. The basic fragments ae «hiefiy 



' Th«- uiiiulxTsfif b|N.-ciiiieii* ;iii<l uliilii* aif tbu6<.- ofilii- colli-rtiuti of the I.ak>- .Siip<-rior divUiuD. 

 LocatioU!- art given from the K>utfae4St it>nieri> of the i>ectioos. in slep- >•( 2.0(0 j>«r mile. 



