THE NATURAL HISTORY SURVEY. 2g 



forms of chalcedony, usually chert, rarely agate. The chert is 

 conchoidal and somewhat splintery in fracture, being more 

 splintery as the proportion of impurities increase. These im- 

 purities are chiefly varying proportions of calcium carbonate, 

 which may become so abundant as to pass by insensible 

 gradations into limestone. Other impurities are hydrated sili- 

 cates of aluminum and small amounts of carbon and iron 

 compounds. Chert occurs in great quantities in the limestone. 

 Often the nodules are so numerous as to ruin the limestone 

 for building purposes, since the point of contact between the 

 chert and the limestone is a line of weakness. When the 

 chert nodules are very abundant the stone is unfit even for the 

 limekiln. Walking along the spoil banks of the drainage 

 canal it becomes evident to the observer that the easily dis- 

 solved limestone being carried away would leave a great mass 

 of a difficultly soluble chert, and this mass, when waterworn 

 and acted upon by heat and cold would soon form gravel 

 banks such as those common to the lake shore. The 

 presence of silica in the limestone is probably due to plants 

 and animals which, while living in the waters from whence 

 came the limestone, secreted silica in their shells and skele- 

 tons. The remains of many of these animals and plants sea- 

 weed, desmids, diatoms, sponges, rhizapods can'be distin- 

 guished. The right hand specimen in PI. VII, fig. I shows some 

 of the forms of life which contributed to the formation of these 

 rocks. The most abundant forms are cylindrical bodies of 

 varying lengths, many of them a quarter of an inch long, 

 some of them are straight while others are bent into bizarre 

 forms. The body-wall is made of silica which is less perfectly 

 crystallized than that found in the body cavities. Other forms 

 may with great probability be classified as radiolarians (Spum- 

 ellaria?) sponges (Astylospongia), crinoids (Pisocrinus, Ste- 

 phanocrinus), bryozoans (Hemiphragma?) A microscopic 

 section under crossed Nicols reveals polarization colors be- 

 tween the different organic forms and in the interior of these. 

 No chert which I have examined has failed to show polariza- 

 tion colors. The oft repeated statement that chert is non- 

 crystalline is a wrong one. Examination of the nodules sheds 

 light upon the manner of their formation. Waters laden with 

 silica deposited their burden quite rapidly at first on the bor- 

 dering walls of the cavities in which the nodule was forming. 

 This rapid deposition produced the least pure and least well 



