ENDLicH.] ORIGIN OF COLORADO ERUPTED ROCKS. 269 



the points and along the lines of least resistance, jjnd fused rocks would 

 be ejected within the same region successively for a long period of time. 



All the phenomena which must be the inevitable resultants of such 

 conditions and occurrences as are assumed above to take place, have 

 been observed within the ronge of the study of vulcanicity. 



As, perhaps, the primary conditions, we must regard the accumula- 

 tion of sediment at any one point, or along a certain line. This will be 

 most readily effected along the edges of large bodies of water. There 

 we find to-day the best development of volcanic activity. Eruptions 

 that belong to past geological periods may be found to have taken 

 place in regions which were similarly situated. Near the borders of 

 large bodies of water this liquid has the best opportunity of penetrating 

 through rocks to the greatest depths. Hence it is there that we will 

 find the best supply of so powerful a solvent agent. According to 

 Scheerer* the '' presence of 5 or 10 per cent, of water may suffice, at 

 temperatures approaching redness, to give to a granitic mass a liquid- 

 ity partaking at once of the character of an igneous and aqueous fusion." 



Eruptions have been observed, showing the action of some suddenly- 

 developed force, and islands have thus been formed, some of which have 

 remained to the present day, while others, again, have disappeared. 



As a rule, the rocks forming lavas do not consist of hydrated miner- 

 als. Exceptions are formed by many occurrences of quartz, however. 

 Nearly always the matrix of semi-opal or opal has proved to be of un- 

 doubted eruptive origin. Here, then, we have an instance in which the 

 water was retained as a component part of the primarily anhydrous 

 mineral. It is highly probable that minerals — if retaining any individu- 

 ality in the fused magma — and the magma itself have lost their water 

 at that point when it was rapidly converted into steam. Generally, 

 lavas show more or less porosity. This characteristic feature is evi- 

 dently the result of gaseous enclosures. A large percentage of tbe en- 

 closed gases must be referable to atmospheric air. But the vesicular- 

 ity is often observed at such points where air could scarcely have had 

 any access to the lava. Without assuming too much, then, the vapor 

 ])roducing this porosity may, in many instances, be referred to steam. 

 Any gas of specific character would probably have produced such alter- 

 ations of the immediate surroundings of its enclosing mass that its for- 

 mer presence could even now be detected. 



The views above given were suggested by a study of the eruptive 

 rocks in Colorado, and to my mind they offer perfect agreement with 

 the observations that led to their present form. 



ORIGIN OF THE ERUPTED ROCKS OF -COLORADO. 



In the year 1873, I was struck with the ultimate similarity of some of 

 the mctamorphic rocks and the trachorheitic eruptions observed in Col- 

 orado. I then published, without further elaboration, the opinion that 

 I regarded the latter simply as the result of the fusion of granites f. At 

 the same time. Dr. Peale came to similar conclusions. He suggests,^ 

 whether the eruptive granites of the Elk Mountains might not prop- 

 erly be regarded as a remelted metamorphic granite, and points out the 

 possible establishment of a direct connection between the trachytes and 

 the granites of that region. 



The more I had an opportunity to study the eruptives of Colorado, 



*Am. Jour. Sci. and Arts, T. S. Hunt, vol. 4, p. 26. 

 tRep. U. S. Geol. Surv., 1873, p. 350. 

 t Ibid., p. 261. 



