494 Prof. T. Sterry Sunt — Elements of Primary Geology. 



2. The history of the origin of all these rocks is one of evolution 

 through successive differentiations. The transformations of the 

 primitive igneous material of the earth's crust, through the action 

 of air and water, aided by internal heat, present a mineralogical 

 evolution not less regular, constant and definite in its results than 

 the evolution apparent in the organic kingdoms. The nature of 

 this very complex operation, and its various stages, the writer has 

 elsewhere endeavoured to set forth at length, and the object of the 

 pi'esent note is to state concisely, and in an elementary form, some 

 conclusions and generalizations reached after forty years of study ; 

 referring the reader for details to his recently published volume 

 entitled Mineral Physiology and Fhysiography. It may, however, be 

 said in a few words that the protoplasmic mineral matter — which we 

 may define as the superficial portion of a cooling globe solidifying 

 from its centre — was, so soon as its surface became sufficiently re- 

 frigerated, the subject of a double series of processes. These included 

 (1) the removal from its mass by solution, through, permeating 

 waters, of the elements of the Neptunian, or as I have elsewhere 

 named them, the Crenitic, crystalline and colloidal or porodic aggre- 

 gates ; (2) the modification of the residue by this watery lixiviation, 

 removing silica, alumina and potash, and when sea-water intervened, 

 bringing in lime, magnesia and soda in exchange ; and farther by the 

 crystallization and partial differentiation through the influence of 

 gravity, and by eliquatiou, of more or less altered portions of this 

 primitive mass, resulting in the production of various types of 

 properly igneous or plutonic rocks. 



3. The continued removal therefrom of the elements of the great 

 succession of crenitic rocks, which were laid down upon its surface 

 alike as indigenous and endogenous masses, caused a progressive 

 diminution in volume in this water-impregnated primitive plutonic 

 stratum; which must not only have produced a profound corrugation 

 of its earliest crenitic envelope, but through the succeeding ages of 

 the duration of this operation, have given rise to that generally 

 folded and inclined attitude, with frequent discordances, which 

 characterizes the Primary stratified rocks, while it is only occasional 

 and accidental in Secondary strata. With the decline of crenitic 

 action the weight of accumulated strata apparently brought aboiit the 

 plutonic and pseudoplutonic eruptions ; which appear to have been 

 rare in the earlier periods, but in later times to have replaced the 

 previously continuous upward transfer of matter from the primitive 

 plutonic substi-atum by the process of aqueous solution. In thus 

 indicating what must have been in early periods a potent agent in 

 producing corrugation and disturbance of strata in the crenitic rocks, 

 it must not be forgotten that from the beginning to the present time 

 the contraction of the terrestrial anhydrous nucleus from secular 

 refrigeration has also doubtless been a factor therein not to be over- 

 looked. 



4. The crenitic masses present in successive ages marked variations 

 in mineral type, which furnish the basis of a chronological classifi- 

 cation having regard especially to the relative proportions of silica, 



