64 DR. THOMAS STERRY HUNT ON THE 



by diageuesis into crystalliue hydrous or anhydrous species, are questions for farther dis- 

 cvission. The range of temperature throvigh which we have noted the crystallization of 

 chabazite, and the association of orthoclase by contemporaneous or subsequent crystalliza- 

 tion with hydrous species like zeolites and chlorite, lead us to conclude that for the hydrous 

 and anhydrous aluminous double silicates alike, a considerable range of temperature is 

 permissible. In any case, we find nothing in the conditions of the formation of zeolitic 

 minerals in the past, any more than in modern times, incompatible with the existence of 

 organic life. 



§ 12*7. The x)henomena of exoplutonic action, or so-called vulcauicity, though relegated 

 to a secondary place in the crenitic hypothesis, are yet, as we have said, of great importance 

 and significance, and are by no means simple. They were, according to our hypotUesis, 

 confined in early times to fissure-eruptions of the underlying primary stratum. This, 

 although in the course of ages it has suffered a gradual change from the ceaseless crenitic 

 action, which has removed from it the elements of the various series of crystalline rocks, 

 including the primitive granitic and gneissic series, probably still retains in the lower 

 portions somewhat of its original constitution. A second phase in the history of exoi)lu- 

 tonic rocks, already foreseen by the Huttonians, here presents itself for our consideration. 

 The more deeply buried portions of the primitive crenitic deposit, must themselves have 

 been brought within the influence of the central heat, and, iDcrmeated as they were by 

 water, have suffered a softening which permitted them, as a result of subsequent move- 

 ments of the crust, to appear again at the earth's surface as exoj)lutonic or exotic rocks 

 of the trachytic or granitic type. 



We can hardly suppose the displacement, either of the primary igneous mass or of the 

 early granitic deposits, to have been attended with the evolution of permanent gases, such 

 as attend modem volcanic eruptions and are to be ascribed to the action of subterraneous 

 heat on more recent deposits, including carbonates, sulphates, chlorids and organic matters. 

 Such materials, when mingled with silicious and argillaceous sediments, and brought by 

 local accumulation and depression within the heated zone would give rise to the various 

 gases which characterize the volcanic eruptions of recent periods, in which, however, the 

 materials of the underlying primary and crenitic layers apparently intervene. 



By thus ascribing a three-fold origin to the products of exoplutonic action, it becomes 

 possible to classify and harmonize the apparently discordant phenomena of eruptive 

 rocks. While the typical basalts and related basic rocks would be derived from the primary 

 igneous stratum, and the trachytic and granitic rocks from the earlier crenitic deposits, the 

 more fusible portions of the later transition and secondary strata may have furnished their 

 contingent, not only of gases and vapors, but of lavas and volcanic dust. 



§ 128. The history of the origin of crystalline rocks is the history of the origin of the 

 mineral species which compose them. The crystalline masses are essentially made up 

 of a few groups of species. Various feldspars and occasional zeolites, some of which 

 apparently occur as integral parts of rocks chiefly feldspathic, form a great central group. 

 On one side of these are the aluminous double silicates, represented by basic species like 

 garnet, epidote, magnesian micas and chlorites, all with an excess of protoxyd-bases ; 

 while, on the other hand are the aluminous double silicates of the muscovitic and pinitic 

 groups, in which the diminished proportion of the i^rotoxyd-bases prepares the way to the 

 associated simi^le aluminous silicates, pyrophyllite, andalusite, cyanite, etc. To these 



