THE EASTERISr AREA. 383 



W.; 7453 (slides 2048 and 2049), 1 :.()(» N., 1550 W., Sec. 23, T. 47 X., 11. 44 W., 

 Michigan. 



The rocks are like 2. Upon the weathered surface tlie more resistant pebbles 

 protrude with oval outlines. When tlie rock is freshly Tract n red the pale grayish- 

 green, rounded pebbles are well dctincd in their darker colored matrix. 



In tliin section the smaller fragments iind their relations to tlie backgronnd are 

 precisely as iu 2. This background is liner grained, and appears to consist of very 

 finely crystalline quartz, mingled with the finest sort of debris from material like 

 that composing the fragments. Tliis background and the smaller fragments are 

 arranged in a roughly stratiform way, whicli must indicate stratification or a flowage 

 structure. The sections contain abundant larger fiagments, wliicli vary in character 

 from those having an almost wholly devitrified glassy background containing much 

 altered tabular plagioclases to tliose which have a holocrystailine base in which is 

 found many comparatively fresh augites and porphyritic crystals of ])lagioclase. 

 The plagioclase has often wholly altered to white mica or has been replaced by calcite. 

 Since the above pebbles contain augite, they are proi)erly angite porpliyrites. 



7. Greenstone-conglomerates north of (i, near mi<ldle of l)elt. Siiecimcns !i.'t.'57 

 (slide 3022), 75 N., 1500 W.; 7405 (slide I'JIU), 7400 (slide 1!I17), 74(i7 (slide 1!»18), 140 

 K, 1500 W.; 9330 (slide 3021), 270 N., 1.500 W.; 74(i.S (slide 191!l), L'!t(l N., 1.5(10 W., 

 Sec. 14, T. 47 K, R.-44 W., Michigan. 



In places the conglomerate is free from pebl)les, and is then dark giavish- 

 green, tine grained, and*schistose. The i)ebbles, wiien present, are all of the green 

 massive variety described in 2. They vary in size fi-oni tliose so small as to be lost in 

 the matrix to those several inches in diameter. The conglomerate is tiiickly studded 

 with the j)ebbles, which are best seen upon the weathered surface. 



The matrices of the sections are of two varieties; in one, finely crystalline ipiartz 

 is the predominant constituent, in the other it is subordinate in (piantity. In this 

 latter ]>liase with the (piartz there is mingled very abinidant chlorite, much ainoi|)lions 

 gray material, and many i)artly altereil tabular jilagioclases. The phase of t!ie ma-, 

 trix rich in (piartz also carries these minerals, although in less ((uantity. Contained 

 in both varieties of matrix are, scattered somewhat sparsely, large well ronnded 

 fragmental grains of feldsjiar and (piartz, the latter being sometimes hunched into 

 areas of some size, the constituent grains of which are usually (Milarged. 'I'he peb- 

 bles, as in the previously described conglomerates, are fragments of alteii'd basic 

 eruptives. Commonly this alteration has extended very far, so that they arc now com- 

 posed of chlorite, epidote, and partly dcconiixised tabular jilagioclases as cliief con- 

 stituents, and with these rather abundant titaiiite and some secondary (piaii/. The 

 pebbles vary from (piitewell rounded to those as irregular in form as in L'. ;\Iany of 

 them are cut, into or even apparently, dissevered by the ramiiyinj; (piaitzioe matrix. 

 The well rounded chaiact-er of the (piartz grains in a matrix of nonliagmental (piartz 

 would seem to be evidence that these rocks were formeil under water. 



