ORIGIN OF CHERT IN LIMESTONES. 817 



It is well known that in many limestones, dolomites, and marbles are 

 nodules and belts of opal, semicrystalline opal, chalcedony, finely crystal- 

 line quartz, or coarsely crystalline quartz, or various combinations of these 

 materials. Such rocks are called cherty limestones, cherty dolomites, and 

 cherty marbles. The siliceous material may constitute separate nodules. 

 The nodules may unite at their ends so as to form layers of varying' thick- 

 ness within the limestone. The chert may be in bands with no evidence of 

 a nodular character. Bands of chert usually have a lenticular form. The 

 lateral extent of the lenses is likely to be in direct proportion to their 

 thickness. The layers, which are a few centimeters in thickness, are usually 

 only a few meters in lateral extent. The less numerous layers, which are 

 several meters in thickness, may have a lateral extent of a number of 

 kilometers. At the horizon at which one lens dies out others may appear, 

 and thus the large lenticular bands at a definite horizon be analogous in 

 their relations to magnified nodules. Rarely the masses of chert are many 

 meters thick and many kilometers in length, constituting considerable 

 formations. So far as such formations occur, they are treated on a subse- 

 quent page. The limestone or marble formations containing nodules or 

 thin layers or bands of chert as subordinate members, and usually having 

 no great lateral extent, are known as cherty limestones or cherty marbles. 

 The majority of those who have worked on chert in the cherty lime- 

 stones, including Wallich, Sorby, Sollas, and Hinde, regard the silica of 

 the chert as having been deposited by organisms simultaneously with the 

 limestone. Of these organisms siliceous sponges appear to be the most 

 important, although radiolaria and diatoms are of much consequence. All 

 these animals live under essentially the same conditions as the limestone- 

 building animals. On the whole, in proportion as rocks are old, the evidence 

 of the organic origin of the silica becomes less and less. Oftentimes in the 

 Paleozoic and pre-Paleozoic cherty limestones no remnants of sponge, 

 diatom, or radiolarian can be found. But this does not show that organisms, 

 were not as important in these formations as in the later formations, since 

 there has been a longer time for their obliteration through recrystalliza- 

 tion. On the whole, I incline to the belief that the silica of the cherty 

 limestones and marbles is- mainly due to organic precipitation, although 

 this source is supplemented by contributions furnished by ground waters, 

 as explained below. 



mon xlvii — 04 52 



