MIDDLE CAMBRIAN. 13 



tlie lamellae, marked by abundant parallel mica scales, pass directl y through the nodule. 

 In these the nodule is apparently due to the siliciflcation of a portion of the stratified 

 mud, which originally differed but slightly, if at all, from that which formed the mass 

 of the surrounding shales. 



MODE OF OCCURRENCE OF THE SILICEOUS NODULES. 



The description of the mode of occurrence of the siliceous nodules is 

 based largely on notes prepared by Dr. C. Willard Hayes, who mapped 

 the areal geology of the region and studied the rocks with great care. I 

 made a hurried trip to the region in which the nodules occur, in company 

 with Dr. Cooper Curtice and Mr. S. W. McCallie, of the Geological Survey 

 of Alabama, in the summer of 1895. 



The shale from which the "cobbles" or siliceotis nodules were derived 

 is finely laminated, greenish, yellowish, or gray at the surface, and gener- 

 erally bluish-black below drainage. They are found in several narrow 

 bands, extending northeast and southwest, near the center of the Coosa 

 Valley. These alternate with other bands of brown argillaceous shales and 

 bands of interbedded shale and limestone. Although the evidence is far 

 from conclusive, it appears probable that the cobble beds belong in the 

 highest, the limestones in the intermediate, and the brown shales in the 

 lowest division of the Coosa Valley formations. If this is the case, the 

 cobble beds correspond with beds of greenish micaceous sandstone along 

 the southeastern side of the valley. In some places these sandy beds can 

 be traced directly into the cobbles through all intermediate gradations. 

 The sandstone beds become thinner, the grains finer, and the siliciflcation 

 less uniform, resulting at first in thin plates slightly more resistant than 

 the mass of the shale, then broad, thin lenses, and finally the flat ellipsoidal 

 cobbles. Also, the cobble beds probably correspond with the shales on the 

 northwest side of the valley, which cany thin plates of limestone and calca- 

 reous nodules, the latter closely resembling the siliceous nodules in which 

 the medusae are found. They vary considerably in the amount of calcare- 

 ous matter which they contain, from nearly pure to highly siliceous 

 limestone. The silica appears to be original and not secondary, due to 

 replacement since the rock solidified. 



If the above con-elation is correct, there is a marked change in the 

 composition of contemporaneous deposits from southeast to northwest — 

 from silicified micaceous sandstone, through argillo-siliceous shales contain- 

 ing siliceous nodules to argillo-calcareous shales containing calcareo-siliceous 





