IOWA ACADEMY OF SCIP:NCES. 29 



the Des Moines river there are seven that do not occur in the Iowa, while 

 in the latter stream there are twenty-one forms that are not found in the 

 former; twenty-six species are common to both rivers. Of the latter, four 

 are rare in the Iowa but abundant in the Des Moines, while two are rare in 

 the last mentioned water course and common in the eastern stream. 



Now the molluscan fauna of the Iowa is identical with that of the Minne- 

 sota river, suggesting that an intimate connection may have existed, at a 

 period not very remote, between the latter stream with some oue of the 

 drainage basins of eastern iowa. That the connection was probably of a 

 comparatively recent date is shown by the distribution of the living 

 Unionidie in the upper Mississippi valley which points strongly t© the wide- 

 spread influence of certain peculiar agencies during glacial times which 

 modifled the former range of the mollusks of the region. The present topo- 

 graphy, however, of southern Minnesota, does not seem to exhibit any direct 

 indications of such a relation as is above alluded to, except in the central 

 part. But it is probable, as has been urged by Chamberlain, McGee and 

 others, that during the glacial period the elevation above the sea level of 

 the region under consideration was very different from that of the present 

 time. The objection raised by tlie previous statement therefore loses most 

 of its force. 



The persistence, with such slight structural modifications, of the mem- 

 bers of the UnionidiB for the long period of time that must have elapsed 

 since the close of the Cretaceous appears to indicate a high antiquity for 

 this type of molluscan life. But since so very little or nothing is known 

 concerning the internal characters of the shells of the Paleozoic lamelli- 

 branchs, it is very probable that a number of other Unio representatives 

 will be found among forms already described under genera not at all 

 related. On the other hand future research will doubtless bring to light 

 new types connecting more closely the family with others. In this con- 

 nection it is of interest to note that Whiteaves has latelj' described some 

 lamellibranchs from the Coal Measures of Nova Scotia which with little 

 doubt possess characters which would cause great difficulty in the attempt 

 to separate the forms from typical Unio. 



PROCESS OF FORMATION OF CERTAIN QUARTZITES. 



BY CHARLES R. KEYES. 



( Abstract. ) 



In the extreme northwestern corner of Iowa there is a small area of 

 very hard, thoroughly vitreous rock, which has been known for more than a 

 quarter of a century as the Sioux quartzite. The mass is also well exposed 

 in the adjoining portions of Minnesota and South Dakota. The Sioux 

 "granite," as it is now locally called among quarrymen, is of considerable 



