THE NATURAL HISTORY SURVEY. IQ 







ara limestone can be classed as a native element, and it is the 

 only one of the elements which is represented in the limestone 

 rock of this region. 



Gold. An. Regular. 



Cook County does not escape the universal search for 

 gold. Periodical reports of gold discoveries near Chicago 

 appear in the papers. Material supposed to contain gold is 

 being repeatedly sent to our assay laboratories. This supposed 

 gold represents a great variety of substances. One of the most 

 common is pyrites. Less commonly the samples are pieces of 

 limestone that have been scratched by the copper nails in 

 shoes. Sometimes they are decomposed mica, which has a 

 reddish yellow color and is infusible in an ordinary fire. A 

 blow of the hammer would place pyrites out of the question, 

 for it is brittle and hard, emits sparks when struck, and gives 

 the odor of burning sulphur. A drop of nitric acid would eat 

 away the copper nail scratches on the marked pebbles. An 

 attempt to wash the decomposed mica would show its light 

 weight, for the scales are so light as to be with difficulty sub- 

 merged, while gold is nineteen times as heavy as water. 



Gold is present in the region, but in quantities gauged to 

 the assayers balance rather than to the prospectors pocket- 

 book. It is found in the lake shore sands in quantities sufficient 

 to give a trace. Some specimens of cabinet size have been 

 found in the drift. They have come from the north, possibly 

 near the straits of Mackinac. Worthen reports the finding of 

 gold flakes in drift material (A. H. Worthen, Economical 

 Geology of Illinois, Vol. I. p. 27 et seq.). One of the best sam- 

 ples seen by the writer was found at Willow Springs during 

 the digging of the Drainage Canal, and is now in the posses- 

 sion of Mr. L. E. Cooley. The pebble is a piece of tourmaline 

 granite, and was deeply buried in the gravel. The gold flakes 

 are scattered through the mass, chiefly in the tourmaline. An 

 occasional octahedral plane is evident on some of the string- 

 ers. When attempt is made to remove the flakes, it becomes 

 evident that they are not plastered on by the finder, as they 

 are connected with underlying fibers that permeate the mass. 

 They are malleable, ductile, 2.5 in hardness, and 19 times as 

 heavy as water. A flake scorified, cupelled, treated with micro- 

 cosmic salt, and immersed in concentrated nitric acid gave no 

 reactions for silver. 



