THE EASTERN AREA. 409 



The i<K'ks \:iry Iroiii tircciiisli gray to inottlotl urceiiish gray or rod, aro nipdium 

 graiiu'il, niassivc, and hrrak witli a somewhat rotigli fracture, ro-sembliiij;- in this 

 res])ect /il. 



Ill most resj)octs tlic tliiii sections are similar to tliose of 51, the essential dif- 

 ference between the two heiii^' that the matrices conlain a large amount of lerrodolo 

 mite. The it'lafions of the hematite and limonite to the ferro-dolomite are often such 

 as to clearly indicate that these oxides are thiiresult of decomposition of that miueral. 



."").".. ("onglonierate. Specimens 12011 (slide r..''.C4), 12012 (slide .").".G.")), 12800 (slide 

 o4Sl), l(».-)() N., 1000 W., Sec. 28, T. 47 N., E. 42 AV.. Michigan. 



The conglomerate contains numerous white quartz, green si-liist, and worn 

 granite pebbles. No unmistakable greenstone pebbles are seen, although some s]>eci- 

 mens of the conglomerate appear to contain small fragments of basic detritus. At 

 times the conglomerate is so fine grained as to become a quartzite. 



Thin section 5481 is from a red (juartzite pebble. It is composed almost wholly 

 of enlarged grains of quartz of nearly uniform size. Mingled with this (|nartz is a 

 little feldspar. Between these grains are also Alms of limonite. In slide 5.304 the 

 pebbles are from green schist fragments. The green schist is very much altered, and 

 consists mainly of confu.sedly mingled pale green chlorite, much altered feldsjiar, <'a(- 

 cite, or other carbonate, sericiite or muscovite, and leucoxene. Whether it is ;in 

 altered eruptive it is impossible certainly to say. The matrix of the conglomeratic is 

 of the most comjilex nature, being comjiosed of finely crystalline and coarse frag 

 mental i)ortions. The coarse grains are quartz and feldspar. TIu\v have been in 

 ]iait somewhat rounded, but are mostly angular. The iiuartz grains are <;l'leii quite 

 widely enlarged, and frequently the enlargements gradually merge into tiie matrix. 

 The fragments of feldspar are as numerous as the (piartz, and include ortlioclase, 

 microcline, and plagioclase. Some of the latter iiartidcs have excessively iine twin- 

 ning. Many of the particles of feldspar are plainly enlarged. In some cases, particu- 

 larly with ortlioclase, tlie added jiortions •extinguish simultaneously with the cores 

 but in the striated feldspars the added portions, which entirely encircle the cores, 

 extinguish wholly with one set of the bands. This may mean that tlie polysynthetit; 

 twinuiug of the original feldspar has been due to some cause which acted upou the 

 kernels of feldspar before they were deposited in the present rocks. A few frag- 

 ments of mica are also found. These large areas are contained in an excessively fine 

 grained groundinass, the chief ciuistituent or which is crystalline aud half aimnplions 

 silica, mingled with chlorite, sericite, uv kaolin, and many minute rectangiila and hex- 

 agonal crystals of hematite. The groundmass contains al.so a few laigic areas of ferro- 

 dolomite. 



