794 FRANCIS M. VAN TUYL 



and to discount the importance of the algal theory. To quote from 

 him: "The dead algal cells in the Salt Lake oolite are regarded as 

 cells which had selected the oolite as a point of attachment. They 

 became imprisoned within it by the further accretion of aragonite 

 by chemical precipitation." He suggests that the decay of the 

 attached algae furnishes Na 2 C0 3 which acts as a precipitating agent 

 and thereby aids the growth of the oolite. 



As regards the importance of algae in the production of the 

 oolites of Great Salt Lake, future studies may be expected to throw 

 additional light on the problem. Microscopic examination of 

 these by several investigators has failed to reveal any indications 

 of algal structure in the calcareous grains themselves. On the other 

 hand, they exhibit highly developed radial and concentric structure. 



THE PRAIRIE DU CHIEN OOLITE 



Some time ago the writer had occasion to examine microscopi- 

 cally a siliceous oolite which marks the base of the Ordovician in 

 northeastern Iowa, and found to his surprise that the oolite grains 

 of this showed undoubted algal structures. The bed in question 

 constitutes the so-called transition member between the Prairie 

 du Chien dolomite and the Saint Croix sandstone. With reference 

 to this bed Leonard, in his " Geology of Clayton County," says: 



The lower Magnesian is not marked off sharply from the underlying Saint 

 Croix, but there is a transition from the one to the other through from fifteen 

 to twenty feet of calcareous sandstone or siliceous oolite. The rock is com- 

 posed of clear rounded grains of quartz cemented by lime carbonate. In 

 some beds this cementing material is quite abundant, in others there is only 

 enough to hold together the grains. The ledges vary in thickness from a 

 few inches to two or three feet. This siliceous oolite is well exposed in an old 

 quarry in the river bluff one and one half miles above North McGregor. The 

 transition beds are also seen in the section at Point Ann, just below McGregor. 

 Here there are alternating layers of sandstone and limestone and some oolite 

 similar to that described above. 1 



A bed of similar character and thickness has been described by 

 Calvin 2 as occurring at the same horizon in Allamakee County, 

 which lies directly north of Clayton. The writer has examined 



1 Iowa Geol. Survey, XVI (1905), 239-40. 

 ■ Ibid., IV (1894), 61. 



