36 DR. THOMAS STERRY HUNT ON THE 



and also in more recent times, through the action of thermal waters ; it being shown, from 

 the association of zeolites with feldspar and quartz in nature, that all these are sometimes 

 formed contemporaneoixsly from aqueous solutions, and also that many feldspathic veins 

 and masses have probably had a similar aqueous origin. 



IV. — 18Y1. The subject of granite veins, farther discussed, and the mineralogical 

 similarity between these endogenous masses and the indigenous gneissic and granitic 

 rocks insisted upon. 



V". — 18*74. The argument reiterated, that the conditions under which the primitive 

 granitic and gnessic rocks were produced were essentially similar to those of the granitic 

 veins of the later crystalline schists, and that these conditions are reproduced to a smaller 

 extent, in later times, in the formation of zeolitic minerals : finally, that the gneisses and 

 bedded granites are to granitic veins what beds of chemically-deposited limestone and 

 travertine are to calcareous veins. 



VI. — 1880. The definite assertion of the aqueous origin of stratified crystalline rocks, 

 coupled with the rejection of the doctrines of metamorphism and metasomatism in explain- 

 ing their origin, and the assertion of their pre-paleozoic age. At the same time, the proba- 

 ble intervention of clays, from the subaerial decay of feldpars, as a source of certain crystal- 

 line aluminous silicates is suggested. 



VII. — 1884. The definite assertion is made that the ancient crystalline rocks were 

 generated either directly from materials brought to the surface by subterranean springs 

 from the primary igneous rock, or, as was the case in later times, by the reactions of these 

 materials with the products of subaerial decay. These latter included clays from feldspars, 

 and dissolved magnesian salts formed by the action upon sea-water of magnesian car- 

 bonate set free in the atmospheric decomposition of basic rotk erupted from the primary 

 stratum. Thus, while what may be called the Primitive crystalline rocks were wholly 

 crenitic in their origin, the soluble and insoluble results of the subaerial decay, alike 

 of basic exoplutouic matter, and of the older crenitic rocks, contributed to the formation 

 of the later, or Transition crystalline schists. 



III. Illustrations of the Crenitic Hypothesis. 



§ *12. The crenitic hypothesis, which has been proposed in the second part of this 

 essay to account for the origin of the granites and crystalline schists, conceives them to 

 have been derived, directly or indirectly, by solution from a primary stratum of basic rock, the 

 last congealed and superficial portion of the cooling globe, through the intervention of 

 circulating subterranean waters, by which the mineral elements were brought to the sur- 

 face.' This view not only compares the generation of the constituent minerals of the 

 primitive rocks with that of the minerals formed in the basic eruptive rocks of later times, 

 but supposes these rocks to be extruded portions of the primary stratum which, though 

 more or less modified by secular changes, still exhibited after eruption, though on a limited 

 scale, the phenomena presented by that stratum in remoter ages. The study of these rocks, 

 and of their accompanying secondary minerals, which may be properly described as the 

 secretions of these rocks, will therefore be found very important as illustrations of the 

 crenitic hypothesis. 



