450 DALE— CAMBRIAN MANGANESE DEPOSITS OF [April 25. 



DiAGENETic Structures 



Banded Structures. — By referring to the description of layer 

 219 A 7 we see that it is a red manganiferous shale with green and 

 brown jaspery bands which may be rather uniform in thickness 

 and may alternate with each other. The green band predominates 

 over the brown so that the greater alternations occur with the green 

 and red bands. Throughout the red shale are numerous nodules 

 of the green and brown jaspery carbonate-oxides of manganese and 

 within the bands themselves are nodular and concretionary forms. 

 The alternating banded and concretionary forms within this bed 

 would indicate alternating conditions of precipitation followed by 

 diagenetic segregational processes which resulted in the formation 

 of nodules and lenticles. Very thin and interrupted laminae of the 

 red band are found with the green bands. The green and brown 

 bands often occur intergrown with each other. From these ob- 

 servations it would seem that these banded structures were evidence 

 of alternate periods of precipitation and that they have assumed 

 their present indurated and concretionary nature by segregational 

 processes which were active throughout the diagenesis of the bed. 



Nodules. — One of the most characteristic features of the shales 

 of the Lower Cambrian is the great prevalence of the nodules (Figs. 

 14 and 15). The following suggestion is offered as to the origin 

 of the form of these nodules with the hope that this line of in- 

 vestigation may be taken up in greater detail at some future time. 

 Though various theories have been suggested for the origin of 

 oolitic spherules and nodules, in general, along organic and inor- 

 ganic lines, nothing of a very definite nature has been brought out 

 as to the origin of their form. The suggestion that surface tension 

 may be the cause of this form is here made. This peculiar and 

 prevalent nodular character of certain beds was brought about in all 

 probability by the tendency of surface tension to decrease the sur- 

 face during the diagenetic stage. Solutions carrying manganese 

 filtering through muds or nearly consolidated muds or shales would 

 quite naturally under certain chemical and physical conditions have 

 the tendency to decrease the surface tension at the contact of the 

 three physical phases; Hquid, colloid, and solid. Starting with a 



