Carboniferous Algal Deposits. 343 



Algal Deposits of the Kona Dolomite. 



The Kona dolomite is a Lower Huronian formation of 

 the Marquette region of northern Michigan. It consists 

 of " a cherty dolomite interstratified with layers of slate, 

 graywacke, and quartzite, with all gradations between 

 the various mechanical sediments and between these and 

 the pure dolomites. The texture varies from quite fine 

 to very coarse and the color from a white to a dark- 

 brown. The dolomite does not appear to constitute more 

 than a half of the formation and the beds vary from a 

 few inches to many feet." 9 The thickness varies from 

 200 to 700 feet. 10 



In the spring of 1917 the writer had the good fortune 

 to accompany Doctor C. K. Leith and his party of stu- 

 dents on the field trip which constitutes a part of the 

 course in "Lake Superior Geology." Among other 

 things geological, the Kona dolomite was studied in one 

 of its best exposures, this being situated on the shore of 

 Lake Superior just south of the city of Marquette. 



The most striking feature of the dolomite in this 

 exposure is the abundance and large size of the algal 

 ccenoplases which compose it. So far as could be 

 observed, the entire mass of dolomite developed through 

 the progressive plastering of one layer of algal material 

 upon another, the only material of other origin which 

 was observed consisting of insignificant streaks of silt 

 between the algal masses. With the assistance of Doctor 

 W. 0. Hotchkiss a group of colonies having a continuous 

 growth from base to summit was measured. It was 

 found to have a thickness of 22 feet with neither base 

 nor summit exposed and a width of 55 feet with neither 

 end shown. This is the extent of the exposure and the 

 actual dimensions must be much greater. Since the 

 dimensions given are of "coral reef" proportions, it 

 can not be doubted that reefs of algae existed in the Lower 

 Huronian sea of this region. 



So far as the writer has been able to learn, the algal 

 growths are not coextensive with the distribution of the 

 Kona dolomite, this being the only place where they have 

 been observed in the formation. 11 In Huronian times 



9 Van Hise, Bayley and Smith : IT. S. Geol. Surv., Mon. 28, p. 244, 1897. 

 10 Van Hise and Leith: U. S. Geol. Surv., Mon. 52, p. 258, 1911. 

 11 Hotchkiss, W. O. : Personal communication. 



