484 Bibliographical Notice. 
flats of the “Great Plains,” is nearly 110,000 square miles in ex- 
tent, nearly equalling the area of Great Britain and Ireland. It has 
may travel over the upturned edges of nearly all the formations in 
the geologi al i 
for ‘tracing out their relations by studying the junction of the 
changed with the unchanged rocks.” Indeed to the geologist Colo- 
rado is almost encyclopedic in its character, as indeed the magnifi- 
azer reports on the metallic (iro ne, lead, 
silver, gold) and other minerals, including turquoise, coal, albertite, 
, and notices most of the chief lodes and are being 
worked, also the oil-wells and the medicinal a t springs. 
“ The gold- and silver-lodes of this Territory, so far as they are 
observed, are entirely composed of the gneissic and granite rocks, 
possibly rocks of the age of the Laurentian series of Canada. A 
any rate, all the gold-bearing rocks about Central City are most 
distinctly gneissie, while those containing silver at Georgetown are 
both gneissic and granitie. The mountain in which the Baker, 
Brown, Coin, Terrible, and some other rich lodes are located is com- 
posed mostly of gneissic and reddish feldspathie granite, while the 
Leavenworth and M‘Clellan Mountains, equally rich in silver, are 
composed of banded gneiss, with the lines of bedding or stratifica- 
tion very distinct. 
“ There is an important question that suggests itself to one at- 
tempting to study the mines of Colorado; and that is, the cause of 
the wonderful parallelism of the lodes—the greater portion of them 
king one general direction or strike, north-east and south-west. 
We must at once regard the cause as deep-seated and general; for 
