SPANISH MOU>'TAlN. 141 



Tlic li(inil)l('iHlc-aii(lcsitc l)(i(K- mi the edj^v of \)r\ Luke \ nllt-v, at tli« 

 soutlnvi'st hasc of Spanish Mountain, will he discnsscd in tiic cliaiitcr 

 devoted to iuneon.s rocks, wliicli t'orni a most iiniiortant urouj), not onl\- in 

 themselves, Init in coiinection witli simihn- oiithui-sts in Sierra \'allev and 

 elsewhere. ih're at 1 )r\- Lake they present a inarxclous vaiiet\- in color, 

 (lensit\- and texture, Itut on careful studv tlie\ are sliown to l)e closeh' 

 related, with a marked silnilarif^■ in mineral and cliemicai composition. 

 The small lioiK- desii^nated on the map as dacite is sim|il\' an extreme i'orni 

 of the larii'er mass, being characterized 1)\' coiisi(h'ral)le free (piartz and 

 l)iotite, and has much the nature of a pumice, while the main hodv niight 

 l)e (h'siguated more concisely as an andesitic j)earlite. 



North of Spanish Mountain, as elsewhere, the Lone Mcmntain lime- 

 stones pass gradually into those of the Nevada epoch, and with this change 

 the structural h-atures of the region assume new aspects, (piiti^ ditferent 

 from the rest of Mahogany Hills or F"ish Creek Mountains. From Hrusli 

 Creek northward the structure is that of a simple monoelinal ridge, 

 trending ahout north 40° west, with a dip invariahK' to the e;ist. Kising 

 al)ove the (.^)uaternary accunnxlations along the east l)ase of tlie ridge in 

 Spring Valley, at sufticiently frecpient intervals to prove the continuity of 

 strata, oi'cnr exposures of (piartzite heds, confoi'inahK' o\erl\ing the lime- 

 stones. As the latter lieds bear ample testimony of their Devonian age to 

 the very snnnnit, the siliceous strata have been referred to the Diamond 

 Peak horizon of the Carboniferous. Brush, i\Iodoc, and Signal peaks are 

 the culminating elevations along this limestone ridge, which stretches north- 

 ward all the way to The Crate. Along the west base of these ])eaks 

 runs the Modoc fault, extending southwaid froui Ila\- lianch \'alle\-, near 

 The Gate, till lost in the Lone Mountain limestones west of Brush Peak. 

 This fault brings up the Diamond Peak horizon in ju.\ta])osition with the 

 Devonian, leaving the limestone ridge between two nearh parallel belts of 

 (juartzite of the same age, conformable on the east side, but unconformalile 

 on the west. As the line of the fault follows the contact between two dis- 

 similar rocks it is easily traced. North of Signal Peak ei-osion has worn 

 out a deej) ravine along the contact, and still fartlier soutliwaiil the east 

 drainage of Reille^■ (-reek also owes its origin to erosion aloui; tiie same 



