Tarr — Origin of the Chert in the Burlington Limestone. 413 



did and interpreted the condition as indicating superficial con- 

 centration by weathering. 



It is evident from the above brief statements that while 

 there are several views as to the origin of chert, the important 

 ones are the first three given, which should really be regarded 

 as two. The theory which seems to be prevalent at the pres- 

 ent time, judging by the recent text books of geology, is that 

 the silica was originally secreted by organisms and then aggre- 

 gated into the nodular or bedded form through replacement of 

 the limestone by circulating ground waters, either before, or 

 after the consolidation of the limestone. The supporters of the 

 theory that chert is due to chemical precipitation assume that 

 the silica was in the sea water, and that it has been precipitated 

 by some means or other, which they fail to suggest. 



II. The Chert in the Burlington Formation. 

 A. definition or chp::rt. 



In this paper the term chert will include those crypto- 

 crystalline varieties of quartz which are white, gray, or blue- 

 gray in color. Dark gray to black varieties will be called 

 flint, while those which owe their color to iron oxides will be 

 referred to as jasper. 



Chert is defined in our latest text books and mineralogies as 

 an impure flint. The writer wishes to express the view that 

 this much quoted statement is wrong and that flint is an impure 

 chert. It is the excessive amount of organic matter in the 

 flint which gives it its black color, and this foreign matter is 

 absent in the chert. Since all these materials are supposed to 

 be pure quartz it would seem that chert is the purest because 

 of the absence of this coloring matter. 



As to composition chert is practically pure silica, as the analy- 

 ses given below show. The other constituents which are 

 usually present are calcium carbonate, alumina, iron oxides, 

 and a small amount of water, the latter being probably con- 

 tained in the opaline silica present in the chert. 



B. megascopic description. 



Megascopically the chert is dominantly white although some 

 of it is gray or mottled. The mottled appearance is a common 

 feature of the chert and is due to the aggregation of organic 

 matter into small areas and to small aggregates of disseminated 

 pyrite. 



Fossiliferous and non-fossiliferous chert occur. The fossils 

 are still composed of calcite, but in some cases they are entirely 

 of silica. While the greater part of the chert is massive and 



