222 GEOLOGY OF THE EUEEKA DISTRICT. 



and with an undulating surface gradually falls away to the broad valley 

 beyond the limits of the map. Its extension westward is lost beneath the 

 Quaternary. It presents an oval-shaped mass one mile and a quarter long 

 by three-quarters of a mile wide, partially hemmed in on both the north 

 and south sides by ridges of Silurian limestones which rest against it. For 

 the most part it is a coarse grained rock disintegrating readily under atmos- 

 pheric agencies. Much of it might easily pass for granite, but other por- 

 tions possess a decidedly porphyritic structure, especially along the line of 

 contact with the Pogonip limestone on the south. Geologically, a more 

 important body is the bold prominent dike trending approximately north 

 and south for over 2 miles, with a varying width, measuring across its 

 broadest expansion nearly 1,000 feet. At the surface no direct connection 

 can be traced between the main granitic area and the dike, the continuity 

 being broken by an arch or saddle of Pogonip limestone which passes 

 beneath the Eureka quartzite of Wood Cone. This limestone saddle is 

 scarcely more than 300 hundred feet in width. There can be no reason- 

 able doubt that the granite-porphyry dike is an offshoot from the main 

 body in much the same way as the branch dikes described farther on are 

 related to the larger one. A strong- proof of this connection is seen in an 

 isolated exposure of the crystalline rock worn bare by erosion on the sum- 

 mit of the limestone saddle. 



The southern end of the main dike contracts to one hundred feet or 

 less in width and splits up into numerous small dikes ramifying in various 

 directions, becoming finally lost in the limestone. Between the surface 

 outlines of the two walls of the dike there exists a marked contrast. On 

 the one side the western wall curves slightly and regularly in sharply 

 defined lines. On the other the eastern side presents a most irregular 

 outline, caused by numerous branch dikes, offshoots from the main dike, 

 trending in nearly parallel courses in a northeastern direction. They break 

 the regularity of outline, besides fracturing and displacing the limestone 

 beds. Fig. 5 shows the position of the secondary dikes to the primary 

 one and is a reproduction on a reduced scale of a portion of the map. 

 The conventional signs indicate approximately the strike and dip of the 

 adjoining limestone. 



