INTRODUCTION. 



Upon first consideration, it might appear that a report 

 upon the mineralogy of this region could be written as was the 

 ophiology of Ireland in a book which had for chapter ten the 

 single sentence, u There are no snakes in Ireland." But inves- 

 tigation shows that the region is far from being poverty-stricken 

 either in the variety or the quantity of its minerals. 



Although its rocks enclose no stores of precious metals or 

 minerals used as gems or for ornamental work, and although 

 lists of mineral localities do not mention Cook County, still 

 the number of minerals that are found is great enough to ac- 

 complish several desirable results. First, they represent five 

 out of the eight great groups or classes into which the mineral 

 world is divided; second, they illustrate the chemical, crystal- 

 lographic, optic, and other physical properties of minerals; third, 

 they demonstrate the chief methods of investigation; fourth, 

 some of them are of such occurrence as to make contribution 

 to the science of Mineralogy. 



Many of the minerals are beautiful and many useful for 

 practical purposes. They are most interesting in a general 

 way because, as individuals or in a fragmentary condition, they 

 constitute the rocks and soil of the whole region. They are 

 universal, and their quantity is unlimited. In their fragmen- 

 tary condition, reduced to the finest powder, mingled with 

 organic matter, wet with water, and spread over the city streets, 

 instead of appearing an interesting object of study, they dis- 

 gust the passer-by, and worry the city fathers, so that it takes 

 a strong imagination to follow Ruskin's suggestion, and see 

 what minerals could result from separating and recrystallizing 

 the various constituents in ordinary mud. The water in the 

 mud, when freed from all foreign substances, would be spark- 

 ling, pure, distilled water. The carbon which makes the mud 

 black, when properly separated and crystallized, would form a 

 diamond. The silica which is usually present in such large 

 proportions, under suitable conditions, would form a banded 

 onyx or a purple amethyst. Then the clay would be trans- 

 formed into a sapphire or the most precious of all gems a 

 ruby; while the lime might be changed into marble or a calcite 



