26 THE CHICAGO ACADEMY OF SCIENCES. 



granulated material which has solidified with such rapidity or 

 under such unfavorable conditions that it has not been possible 

 for the crystal molecules to arrange themselves in a manner 

 conducive to large growth. After a suitable foundation was 

 established, or after the mother solution was sufficiently cleared, 

 long prisms of marcasite crowded upon each other. The cleav- 

 age of marcasite is always indistinct; fracture uneven; tenacity 

 brittle; hardness 6; specific gravity 4.6 to 4.8, being thus some- 

 what lighter than pyrite; luster metallic; color, greyish, green- 

 ish yellow, to pale bronze yellow, lighter than pyrite; streak, 

 dark greenish grey; diaphaneity as in pyrite; magnetic and 

 electrical properties identical with pyrite. 



The chemical composition, association, and occurrence of 

 pyrite and marcasite are so similar that they may best be 

 treated together. They usually contain small quantities of 

 arsenic, antimony, and sometimes of nickel, cobalt, copper, 

 gold, and thallium. Gold and silver are quite commonly dis- 

 tributed through pyrite in such a finely divided condition as 

 to be invisible to the eye, but yet present in such quantities as 

 to furnish a supply of the precious metals. 



These iron sulphides which are so common in all parts of the 

 world are extremely abundant in the region under considera- 

 tion. In the granite, diorites, and diabases of the drift the 

 most usual form is that crystallizing in the regular system. In 

 metamorphic rocks gneiss, mica schist, amphibolite schist 

 and slates microscopic suspended crystals of either the regu- 

 lar or orthorhombic forms often follow the schistose layers. 

 Sometimes they are so numerous as to form pale bands 

 " Fahlbander "through the rocks. At other times their pres- 

 ence is indicated macroscopically only upon decomposition, 

 when they yield white efflorescences of melanterite. Further, 

 the sedimentary formations the Niagara limestones, and the 

 overlying clays and peat contain both pyrites and marcasites. 

 Iron sulphide, usually marcasite, is a common fossilizing material 

 replacing the skin of trilobites, the stems of crinoids and the 

 shells of mollusks. It forms incrustations on calcite geodes. 

 In clay banks, as that shown in PI. Ill, it occurs in innumerable 

 concretions. A bed of marcasite several feet wide extended 

 along the water's edge for 200 feet at the end of Ridge Ave., 

 Evanston, where the lake encroaching upon the shore had 

 caused fresh exposure of the clay and, washing out the finer 

 materials, had left the gravels and marcasite. A portion of this 



