22 IOWA ACADEMY OF SCIENCES. 



Concerning the ereological age of the Sicux Quartzite a number of different 

 opinions have been expressed. 



From the first mention of this formation by Catlin, in 1837, in connection with 

 the celebrated pipestone quarry, until 1866, when Hall entered the reigion, no sug- 

 gestion was made regarding the age of the flint-like mass. Nicholet visited the quarry 

 in 1838, and gave a very complete description of the rock; but for nearly thirty years 

 no special notice appears to have been taken of the place. Hall, though not seeing 

 all the exposures, concluded that the quartzite beds must be Huronian in age. 

 Hayden, who examined the rocks about the same time, thought he had ample rea- 

 son for regarding them as Triassic or Cretaceous. In connection with his work in 

 Iowa, White was led to adopt Hall's opinion; while Winchell approaching the 

 region from the Minnesota side referred the hardened sand-bed to the Potsdam — 

 Upper Cambrian. Still later Irving expressed his view to the effect that the Sioux 

 Quartzite was Huronian. 



Though very little information bearing directly upon the geological age of the 

 indurated sandstones of the Big Sioux region is yet available, the recent observa- 

 tions on the geology of northwestern Iowa are not without interest. 



The presence hi situ of the undoubted eruptive rocks mentioned and at no very 

 great distance below the surface of the ground in the northwestern part of the 

 State; the existence of diabasic masses in the midst of the quartzite mass itself; 

 the folded and disturbed condition of the strata, all point to the great antiquity of 

 the Sioux formation as compared with other rocks exposed within the limits of the 

 State. The inference is, then, that the rocks under consideration must be very much 

 older than any others exposed in Iowa. 



SURFACE DISINTEGRATION OF GRANITIC MASSES. 



BY CHABLES BOLLIN KEYES. 



Throughout the drift mantled surface of Iowa, glaciated boulders of crystalline 

 rocks are of common occurrence. They vary in size from a foot to more than fifty 

 feet across. These boulders are rounded more or less, globular in form, though 

 often slightly flattened on one side, sometimes on two. When closely examined, 

 the outside is commonly found to be more or less affected by meteoric agencies, 

 but the mterior is fresh and unaltered as a rule. Most of the Iowa boulders are 

 known to be of northern origin, coming from near or beyond the present northern 

 boundary of the United States. In this region the granite, diabase and gabbro areas 

 are usually firm, and but slightly decayed at the surface, the rocks having been 

 planed and scored by glacial action. Passing beyond the limits of the glaciated 

 region an entirely different set of phenomena is presented, and in an area of crys- 

 tallines, the rocks are decayed for many, often one hundred or more feet below the 

 surface, the bi-products remaining in situ until removed by running water. 



It is to a granite area outside of the glacial boundary that attention is directed 

 — an area in central Maiyland some twenty miles west of Baltimore, near the vil- 

 lages of Woodstock and Sykesville. 



The Woodstock granite forms a small isolated patch midway between the two 

 largest granitic masses of the region. Though having a superficial area of scarcely 



