ENDLicH.] ORIGIN OF ERUPTED ROCKS. 263 



ous hypotheses and theories tending to explain the phenomena of dy- 

 namical geology. An outline will be given of the suggestions th&t have 

 offered themselves from the examination of the eruptive rocks of Colo- 

 rado. In case subsequent investigations may prove the inductions here 

 derived from observation to be correct they can readily be applied to 

 other localities. 



In any discussion upon the probable source of volcanic activity, we 

 must take for granted the high temperatures existing within the interior 

 of the earth. These are universally conceded, and amply demonstrated 

 by facts. It is essentially immaterial whether we assume that the in- 

 terior of the earth is in a truly liquid state, or whether we regard its 

 condition as merely plastic. Of these the former hypothesis is incon- 

 sistent, however, with the absence of internal tides, in case we assume 

 the earth's crust to be a thin one as compared to its entire diameter. 

 1^0 observations tend to show that such tides exist. 



Formerly the central liquid or plastic state of the earth's interior was re- 

 garded as furnishing the material for volcanic eruptions. More compre- 

 hensive observations and a more thorough knowledge of the character- 

 istics of volcanoes have led to the abandonment of this theory. It is now^ 

 assumed and the assumption is accepted by most geologists, that certain 

 circumscribed portions beneath the earth's surface that must still be re- 

 garded as integral parts of its crust, are in a condition either approaching 

 or actually exhibiting liquidity. From such portions which may aptly 

 be designated as " reservoirs," is derived the material ejected during 

 volcanic eruptions. No means are at hand to establish, definitely, any 

 depth at which they should occur. From phenomena attending vol- 

 canic eruptions, however, it may be inferred that they are located at 

 a dej)th that must be regarded as shallow when compared with the en- 

 tire diameter of the earth. Seismic action is one of the most charac- 

 teristic results of disturbances of equilibrium occurring within certain 

 regions or zones of the earth's crust. The prevalence of earthquakes 

 at any given locality furnishes a criterion as to the relative condition of 

 underlying portions of the crust. Most frequently they may be ob- 

 served in regions that are the scene of volcanic activity, but they ex- 

 tend far inland from any present point of volcanic eruption. A defi- 

 nite relation may frequently be traced between seismic occurrences of 

 one section and eruptions of the nearest active volcanoes. A rather 

 striking example is afforded by a comparison of the slight earthquakes 

 felt in Switzerland and the eruptive activity of Vesuvius. 



From the act of eruption and from the products thereof we can gather 

 only rather general views as to the origin of and the causes producing 

 it. The ejection of lava is usually accompanied by the discharge of 

 watery vapor, of carbonic and hydrochloric acid gases, and hydrogen, 

 both free and in combination with sulphur and carbon. These elements 

 or compounds have not all entered into the composition of the lava io 

 any extent, but occur independently thereof. Eeducing all of them to 

 their original elements, wo find : 



Hydrogen, 



Oxygen, 



Nitrogen, 



Chlorine, 



Carbon, and 



Sulphur. 



Of these, oxygen, carbon, and sulphur are most frequently observed 

 in the lava itself. Direct oxygen compounds and silicates make up the 



