THE EASTERN AEEA. 379 



silile to distinguish the matrix tVoiii tin- iikiiv schistose and alterod (if the 

 Hows. In those phases of tlie greenstone-('ono-h)nierat(' in wliich tlicre is 

 ahnost no matrix at all and those in which the matrix appears hut to he 

 fine dehris of the same kind as the pebbles and howldei-s — that is, com- 

 posed of ang-ular fragments of very greatly varying sizes which luive been 

 closely )»acked together and sidjsequently cemented bv alterations of the 

 materials contained with the addition of interstitial quartz — it seems jirob- 

 able that the rock is mereh" a heap of volcanic tuff. The volcanic material 

 may have fallen upon the sides of the cone or farther awa}' have been 

 dejjosited in water; in which latter case the debris would have been sul)- 

 jected to Avater action. In those cases in Avhich the matrices exhibit the 

 extraordinary irregular and concretion-like areas, varving into i-oughly 

 stratiform deposits, it would seem that the ashes had accumulated under 

 or at least had been subject to the leaching action of water. 



In the phase of conglomerates in which finel}' crvstalline quartz 

 becomes an important constituent it would seem that the rocks had origi- 

 nated by the mingling of volcanic material with nonfragmental sediments 

 under water. Whether the contained fragfnents fell directlv upon the 

 water from a \olcanic vent or the water was the as'ent which broke down 

 this material from recently formed lava can not positively be deter- 

 mined, but the rounded foj'ms of a portion of the 2:)ebbles and the in-egular 

 forms of others would seem to indicate that both processes mav ha\e 

 fimiished a share of the jiebbles. That this class of conglomerate was 

 deposited under the surface of water is further indicateil li\ the fact that 

 in this nonfragmental quartzose background there is not infrequently con- 

 tained well roiuided particles of quartz and feldspar, the former of which 

 have been enlarged, and in a few cases these rounded quartzo.se fragments 

 attain the magnitude of peljbles. That this peculiar mingled fragmental 

 and nonfragmental material is of the origin indicated is furtliei- evidenced 

 by the jasper-conglomerates in the extreme southern part of the 8E. \ 

 of Sec. 20, T. 47 N., R. 43 W., Michigan. The associati(tns of this rock 

 are explained in chapter viii. it was there seen that the conglomerate 

 gradually passes upward into the nonfragmental sediments — chert and 

 cherty iron carbonate — of the iion belt. There is little doubt that it is ;i 



