MICHIGAN ACADEMY OF SCIENCE. 45 



beddiug- is indicated by vai-ialioii.s in llu> size and i-elalive proportions 

 of the varions <>rains and other cases in Nvliicli it is indicated bv hori- 

 zontal jointing. The rotk is very largel.v composed of quartz and feld- 

 s|.'ar. Sericite and kaolin are ]ir(niiinent in light cohered varieties 

 and chkirite in tlie (hirker. Titanite and iron ores are nsmilly 

 present in snuill (inantity. Tlie feUlspar and quartz grains are often 

 well rounded but quite as frtHpiently angnlar or subangular. 



Closely allied lev the shales and arkoses are the greywackes. The 

 chief recognizable const it nents in these are leldspar. ((uartz, a dark 

 chlorite and a pale colored mica. Less abnndant are small ])articles 

 of ii-on ore and ejfidote. while ])yroxene and amphibole are rare. With 

 the minerals are angular and ronnded rock particles of varions sizes. 

 Rock of this type in some instances is found in massive beds of uni- 

 form character, vei*y line grained and of grey to greenish color. Similar 

 nuiterial forms the matrix of much of the boulder conglomerate. 



The couglonuuate is remarkable for its heterogeneous appearance. 

 Not only are the boulders of a great variety of tyjtes but in nmny cases 

 they show no evidence of arrangement according to size. Frequently 

 one finds boulders a foot in diameter scattered irregularly and sparsely 

 through an aphanitic matrix of shale or greywacke, thus simulating 

 glacial de])osits. In other cases there are thick beds of shale cpiite free 

 from such erratics. There are also beds of boulders of nearly eipial 

 size packed close together and with but little of tine grained matrix, as 

 in an ordinary water accumulation of coarse gravel. In some instances 

 the ai>hanitic beds are distinctly laminat(Ml as in ordinary watm- lain 

 clay, while again similar material forms a compact rock lacking in 

 well developed bedding planes. 



As a general rule the large boulders are well rounded or snbangular; 

 but there are occasional streaks <(nitainin-g markedly angnlar frag- 

 ments. Dr. Colennui found some boulders at (N^balt which show striae 

 and concave surfaces. 



The matrix of the conglomerate, which is often greywacke and less 

 often shale, contains numerons angnlai- particles of (pmrtz, feldspar, 

 chert, and felsites, I'articles of snch shajie are very characteristic of, 

 though they are by no means found only in, glacial debris. 



Where the contact of the conglomerate with underlying rocks has 

 been found there is a m)tewortIiy lack of alteration in the older rocks. 

 If they were deeply disintegrated by siirface weathering the material 

 must have been removed by a very el'licient agent. This again suggests 

 ice action. In a few of the contacts the line of demarcation is less dis- 

 tinct, as is ordinarily the case with unconformities in water lain sedi- 

 ments. Naturally the contacts of the- latter ty])e are not so likely to 

 be found as those of the former. 



There has not yet been found a smooth or striated floor. The basal 

 conglomei-ate, in some cases at least, has been formed in situ and is 

 made up of detritus from the immediately adjacent rocks. 



Prof. Colenmn does not consider that the lack of discovery of a char- 

 acteristic glacial lloor precludes 'the ])ossibility of the material having 

 been placed by ice, and lefeis to well known instances in which snch a 

 floor is lacking. 



The localities s|ieciall\- mentioned by Dr. Coleman are Cobalt and 



