CAMBRIAN GLAUCONITES. 339 



omy of the individual grains constituting these quartzose masses, for 

 they present the usual phases of silica as it appears in this type of rocks 

 everywhere; they are externally well worn and of greatly varying size, 

 from coarse conglomerates down to the constituent particles of the finest 

 shale. In many places a green color becomes quite prominent. The 

 cause of this has not yet been satisfactorily determined; the search for it is 

 in its experimental stage by the authors with the hope of a demonstra- 

 tion in the near future. Here it appears to be due to ferrous osfide; 

 there to a glauconitic mineral ; again the conditions of a chlorite in thin, 

 bright-green plates are fulfilled. The green constituent, in whatever 

 phase it occurs, does not seem to possess any cementing quality; yet at 

 Dresbach, Dakota, and even locally at Winona, there is a coherence far 

 greater than is usual in Minnesota Paleozoic sands. At these places a 

 fine white micaceous mineral is very generally present and is regarded 

 as a kind of binding material. A shaly condition alternates with such 

 sandstone in Winona county.* 



Chemical Composition. — But little can be said touching the chemical 

 composition of the Potsdam sandstone. Several years ago Mr. PL G. 

 Klepper made an analysis of this stone from Lansing, Iowa, in the inter- 

 est of glass manufacturers, with the following result: 



Si0 2 62.93$ 



0aCO 3 1D.0-L 



MgCO, 17.0 



' ';; 



^A\ .- o.oo 



99.04% 



Tins certainly cannot be an average composition of the Potsdam sand- 

 stone of Minnesota. 



Paleontologic Characters. — In fossil forms the Potsdam horizon is com- 

 paratively poor. At Taylors Falls several species have been noted, with 

 fragments of at least three undescribed forms. The rock phases at this 

 locality deserve mention in connection with the types of life preserved. 

 The cementing material of the conglomerate is partly dolomitic rather 

 than wholly arenaceous. The source of the carbonates must lie partly 

 in the shells of the brachiopods and trilobites and partly in the decom- 

 position products of the diabasic pebbles, for scarcely any of the finer 

 ones remain. In percentage of MgO these diabases vary from 2.5 to (i.e.. 

 according to Mr. Sweet. I In secluded hays and inlets animal forms 

 could find the protection from enemies and quiet seas and supply of food 



*Cf. N. II. Winehell; Geologj oJ Minnesota, Final Report, vol. i, 1884, pp. 257 el seq. 

 fGeologj "i the western Lake Superior District. E. 'I'- Sweet: Geologj <>i Wisconsin, vol. iii. 

 1880, p. 360. 



X I.Y I I'.i i.i . ia.ni,. 8oc. A.m.. Vol 3. 1891, 



