108 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



criterion in determining the dividing line between the Beekmantown 

 and the overlying Stones River limestone. These cherts are believed by 

 some students to represent in reality the physical evidence of the uncon- 

 formity between these two great limestone formations. Unlike many 

 other cherts they are not the result of present-day surface weathering, but 

 appear to have been formed in the land interval between the two forma- 

 tions. These cherts occur as a regular bedded deposit and their origin 

 seems to be as follows : 



With the uplift at the end of Beekmantown time, weathering of the 

 exposed limestones took place, resulting, as it does to-day, under favorable 

 conditions, in a soil with more or less numerous chert fragments. Con- 

 tinual exposure to waters bearing silica in solution caused a secondary 

 silicification of these cherts by the formation of rosettes of silica over 

 their surface. The rosette areas continued to grow larger and larger 

 until the characteristic cauliflower shape resulted. The reason for the 

 formation of such rosettes is obscure, but it is a fact that fractured fossils 

 or pieces of chert will develop areas of rosette quartz along the fractured 

 zones if subjected to the influence of silica-bearing waters. All of the 

 chert at the top of the Beekmantown has not undergone secondary silici- 

 fication into the cauliflower form. Fragments of platy chert representing 

 the primary silicification stage may occasionally be noted with fracture 

 zones penetrating into or entirely through them. The water with silica 

 in solution will seep into these fractures and deposit the silica there first, 

 thus starting a growth area which develops into the characteristic rosettes 

 of the cauliflower variety. By this process a small fragment of platy or 

 irregular chert by continual growth of the rosette areas will develop into 

 specimens of the cauliflower variety a foot or more in diameter. 



Apparently the same process occurs in fractured fossils found in cer- 

 tain siliceous shale formations, particularly of Silurian and Missis- 

 sippian ages. For example, a crinoid column of say one-half inch in 

 diameter, exposed to silic'a-bearing waters, will first receive a deposit of 

 silica in its central canal and rosettes of silica will project from each end. 

 The column is fractured by this process and each fracture line then 



