210 



UNSTRATTFIED OR IGNEOUS ROCKS. 



never readied the surface (Fig. 180, b'). In either case they are the 

 evidence of extensive erosion. Sometimes the outcropping dike has 

 resisted erosion more than the inclosing country rock, and the dike is 

 left standing like a low, ruined wall running over the face of the coun- 

 try (Fig. 184, a) ; at other times the country rock has resisted more 

 than the dike, and the place of the dike is marked by a slight depres- 

 sion, like a shallow ditch, or moat (Fig. 184, b). 



Effect of Dikes on the Intersected Strata. — The strata forming the 

 bounding walls of a dike, or with which igneous rocks come in contact 

 in any way, are almost always greatly changed by the intense heat of 

 the fused matter. Limestones and chalk are changed into crystalline 

 marble ; clay is baked into porcelain-jasper, or even changed into 

 schists ; impure sandstones are changed into a speckled rock resem- 

 bling gneiss ; seams of bituminous coal are changed into anthracite, 

 or sometimes into coke. In all cases the original stratification and the 

 contained fossils are more or less completely destroyed. These effects 

 extend sometimes only a few feet, sometimes many yards, from the 

 dike. 



Lava-Sheets. — Dikes outcropping on the face of the country, as 

 already described, are doubtless in many cases the exposed roots of 

 ancient overflows which have been removed by subsequent erosion, 

 leaving only the intruded portion. But in more recent eruptions the 

 overflow or erupted portions still remain. The fused matter has evi- 

 dently come up through fissures and spread out as sheets, and often 



sheet after sheet has been suc- 

 cessively outpoured forming 

 layer upon layer (Fig. 185), un- 

 til the whole surface of the 

 country is deeply buried beneath 

 the flood. The extent and 

 thickness of the lava-fields thus 

 formed are almost incredible. 

 The great lava-flood of the 

 Northwest covers Northern 

 California, Northwestern Nevada, the greater-part of Oregon, Wash- 

 ington, and Idaho, and extends far into British Columbia- and Mon- 

 tana. Its extent is not less than 150,000 square miles, and its extreme 

 thickness where cut through by the Columbia Kiver is 3,000 to 4,000 

 feet. In another place seventy miles distant, the Deschutes River 

 cuts into the same lava-field, making a canon 140 miles long and 

 1,000 to 2,500 feet deep, and has not yet reached bottom. At least 

 thirty successive layers may be counted, one above the other, on the 

 sides of this canon. About a dozen volcanoes overdot this great sur- 

 face. It is simply inconceivable that all this material came from 



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Fiu. 185.— Lava-Sheets. 



