146 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



under which the mineralogical differentiation and the cooling of the magma 

 took place. 1 



Study and comparison of the chemical analyses of the two groups of 

 rocks under investigation demonstrate that the magmas that reached the 

 surface of the earth in this place had exactly the same chemical compo- 

 sition as those which remained inclosed within the sedimentary strata. 

 They prove with equal clearness that the different conditions attending the 

 final consolidation of the ejected and of the intruded magmas affected not 

 only their crystalline structure, hut their essential mineral composition. The 

 most marked illustration of this is in the occurrence of biotite in the two 

 series. In the volcanic rocks of this locality biotite is an essential constit- 

 uent of the more siliceous varieties, and is only rarely found as an acces- 

 sory constituent of the varieties with less than 61 per cent of silica. In the 

 intrusive rocks it is an essential constituent of all the coarse-g'rained varie- 

 ties, even the most basic. In the finer-grained, porphyritic forms it is a 

 constituent of the groundmass to a variable extent. The second most notice- 

 able difference is the presence of considerable quartz in the coarse-grained 

 forms of the basic magmas and its absence from the volcanic forms of the 

 same magmas. 



From these observations, then, we see that in this region there are 

 chemically identical rocks which have distinctly different mineral compo- 

 sitions, but which were once parts of a continuous body of molten magma. 

 We are led, therefore, to the conclusion that — 



The molecules in a chemically homogeneous fluid magma can combine in 

 various ways, and form different associations of silicate minerals, producing 

 miner alogically different rocks? 



1 Stelzner, Alfred, Beitrage znr Geologic und Paleontologie der Argentinischen Republik, Cassel 

 and Berlin, 1885, p. 207. 



"Sie [die Andengesteine] wird mis, wie ieh meinerseits glaube, irumer mehr uud mehr erkenuen 

 lassen, dass die grussere oder geringere krystallinitat eruptiver Gesteine keines wegs, wie man so lange 

 mid so hartnackig bebauptet bat, von dein Alter der letzteren abhlingig ist, sondern lediglicb von den 

 pbysikalischen Umstiinden, unter deucii die niineralische Differenzirung und Erkaltung der gluth- 

 fliissigen Magmen vor sicb ging." 



2 This conclusion is tbe same as that stated by Justus Roth: 



"Es konnen mineralogiscb gauz verscbiedene Gesteine in dieselbe Grnppe gehiiren, denn feurig- 

 niissige Massen von gleicber oder sebr liabe gleicher cbemiscber Zusammensetzung konnen in ver- 

 schiedene Mineralien auseinanderfallen. Die Ursacben, welche diese Erscheinung bediugen, lassen 

 sicb hochstens mutbmassen und mdgeu in Uuterschiedeu des Druckes, dor Temperatur, des ungebenden 

 Mediums, der Unterlage u. s. w. gesucht werden." Die Gesteins-Analysen in tabellarischer Ubersiebt 

 und mit kritiscben Erbiuterungen, Berlin, 1861, p. xxi. 



