Geology mid Mineralogy. 65 



character of it, as the abstract shows, is every way calculated to 

 satisfy the reader. It is topographical, lithological, theological 

 and economical in its range, and enters into full details under cadi 

 of its subdivisions. It has many illustrations in the text and an 

 Atlas of fourteen plates. The abstract, by the author, Mr. 

 vh.ole subject, and from it 



In the latitude of Leadville, 39° 15', the Rocky Mountain chain 

 includes (1) the Front or Colorado Range on the east; (2) the 

 Mosquito or Park Range v ' '" 

 which s" 

 farther 



the latter two ranges is the Arkansas V r alley, 60 miles long and 

 16 wide; and on its eastern side, on the western flank of the Mos- 

 quito flange, is situated the city of Leadville, in longitude 106° 

 17' \V. and 10,150 feet above the sea "between I Ji- K van's and 

 California Gulches at the base of Carbonate Hill." The first dis- 

 covery of ore was made in I860. In 1877 its population was less 

 than 200, its opened mines three, and there were surface scratch- 

 ings; in 1880 it was a city of 15,000 inhabitants, and its produc- 



The Paleozoic rocks of the Mosquito Range have a thickness of 

 4050 to 5600 feet and are more or less folded and faulted. They 



comprise (I) 200 feet of Cambrian or Primordial, chiefly quart z- 

 ytes; (■_') over these, 2oo of Silurian (n-f,it, ( ,r dolotnitic limestone 

 and quartzyte): and (:'.) :',7<>oto t-.'oo n f ( ail .oniferous, which last 

 have t! oo t'eej <>[ limestone, called the hhie limestone, at base and 

 1000 to 1500 at top (Upper Measures), with grits (Weber grits), 

 sandstones and shales, partly calcareous, between. In the Kanab 

 section on the Colorado, the Paleozoic has about the same thick- 

 ness (85 feet of it referred to the Permian) ; but in the Wah-atch 

 section cited, the thickness is :30,000 feet, 12,000 referred to the 

 Cambrian. :Unn to the Silurian and Devonian, 15,000 to the Car- 



Besides these there ai-e eruptive rocks — porphyries and diorytes 

 — mostly Mesozoic in age. The common kind is the white por- 

 phyry, an . consisting of quartz (70 per cent), 

 tchlspar (i he latter occasionally in small rectangular crystals), 



decompo-ed, and the muscovite "is the result , .f t he decomposition 



resent. | he -tioryte i - 



The white porphyry occurs to the south of an east-and- 

 hrongh I.ead\ille, and the other kind north of this line. 

 sheet of the former which lies upon the surface of the 

 tone forms, at the f-mile Creek where is its principal 

 arger portion of a hill 2000 feet high, and thence spreads 

 reaching nearly to Buffalo Peaks. On Iron and Car- 

 ls it has a possihle thickness of over 1000 feet; but 



