INDUSTRIAL PROGRESS DURING THE YEAR 1874. \ xx [ 



Northern California and Nevada as far northward as Brit- 

 ish Columbia and Montana, they cover an area which may be 

 estimated at between 200,000 and 300,000 square miles, with 

 a supposed average thickness of 2000 feet, which, however, 

 attains to over 3500 where the Columbia River cuts through 

 the Cascade Mountains, giving in its gorge a grand section 

 of these rocks. This immense volcanic deposit is here seen 

 to overlie an ancient conglomerate layer, and beds of soil 

 with impressions of leaves and the siliciried roots and stems 

 of plants of tertiary age. The operation by which lavas are 

 discharged from ordinary volcanic vents is by Le Conte re- 

 garded as wholly inadequate to explain the formation of these 

 enormous beds of igneous rock, which he supposes to have 

 been poured out from fissures the great movements which 

 sometimes result in foldings of the earth's crust in other cases 

 producing immense disruptions, followed by extravasation 

 of liquid matter from beneath. These discharges were, how- 

 ever, not homogeneous, and the repetitions of doleritic and 

 trachytic rocks show alternate and successive discharges at in- 

 tervals. Richthofen,from his studies, endeavored to maintain 

 a regular order in the succession of the different kinds of 

 eruptive rocks described by him, but the facts observed by 

 Le Conte are difficult to reconcile with such a view. 



James Blake has recently studied with care the minera- 

 logical and lithological character of these eruptive rocks, as 

 seen in the Puebla Mountains in Humboldt County, Nevada, 

 which consist, on their western side, of these rocks in con- 

 formable layers, varying from twenty to fifty feet or more, 

 and dipping westward at an angle of about twenty degrees. 

 The series, from the base to the summit, measures about 1200 

 feet in thickness. The alternations present great varieties 

 in lithological characters. Among them are a compact doler- 

 ite or basalt, chrysolitic and chloritic dolerites, and a coarse- 

 ly porphyritic one, containing twin crystals of labradorite 

 an inch in length, with crystalline plates of augite. These 

 are interstratitied with trachytes, and toward the top with a 

 vesicular trachytes and a porphyritic obsidian. From the 

 association here observed, Blake concludes that the views of 

 Richthofen with regard to the relative ages of eruptive rocks 

 will not apply to those of the Puebla Mountains. Allport 

 has also studied microscopically the dolerites of the British 



