36 THE CHICAGO ACADEMY OF SCIENCES. 



or bituminous substances. The lustre is vitreous or greasy. 

 Before the blow pipe it is infusible. It is soluble with effer- 

 vescence in hydrochloric acid, even though the acid be diluted 

 to one-third its strength. 



The average constitution of the concretionary limestone 

 at Bridgeport is given in the Economical Geology of Illinois, 

 V. I, p. 103, as being practically the following: 



Insoluble silicates or sand 0.42% 



Oxides of iron and alumina 0.53 



Carbonate of lime 57-54 



Carbonate of magnesia by loss 41 . 50 



Total 100 . oo 



Dolomite. CaMgCO s Hexagonal. 



In crystal habit this mineral closely resembles calcite. 

 All the crystals observed are minute, and composed of rhom- 

 bohedra, R and ^R. They incrust crevices and line cav- 

 ities, often presenting stalactitic forms. In chemical compo- 

 sition they are identical with the rock upon which they rest, 

 but they are without the foreign substances, such as sand 

 grains, clay pellicles, and mica flakes. The calcium or mag- 

 nesium is often replaced partly by iron. Upon oxidation, 

 this mineral becomes brown. At other times in color it is 

 white or reddish or greyish. In luster it is vitreous and trans- 

 lucent. It cleaves as does calcite quite perfectly parallel to 

 the rhombohedron. Its fracture is subconchoidal. Its te- 

 nacity brittle. Its hardness somewhat greater than calcite, 

 being from 3.5 to 4.0. Its specific gravity is 2.8 to 2.9. Frag- 

 ments dissolve slowly in cold hydrochloric acid and do not 

 effervesce save when powdered. Being thus less vigorously 

 attacked by hydrochloric acid than is calcite, it is readily dis- 

 tinguishable from that species. The presence of magnesia is 

 not easily detected by blow pipe tests, nor by physical means 

 of investigation, since dolomite is but slightly harder than cal- 

 cite and but very little heavier. The crystal habit and opti- 

 cal properties of the two species are nearly identical. Con- 

 sequently, in the only determinations which I have made for 

 the purpose of detecting magnesium, I have dissolved the 

 mineral in hydrochloric acid, precipitated the iron by a drop 

 of nitric acid, diluted with twice the volume of water, added 

 ammonia to excess, precipitated the calcium with ammonium 



