58 EOCKS FOEMED THBOUaH IGNEOUS AGENCIES 



intratellurial period, marked by the crystallization of certain 

 constituents while the magma, still buried in the depths of the 

 earth, was cooling very gradually, and (2) an effusive period, 

 marked by the final consolidation of the material on or near 

 the surface. As this final cooling was much the more rapid, 

 the ultimate product is a glassy, felsitic, or sometimes holo- 

 crystalline ground-mass, enclosing the porphyritie minerals, or 

 phenocrysts, formed during the first or intratellurial stage/ 

 Naturally the deeper-lying portions of an effusive mass, those 

 forming the under or lower portions of deep lava streams, will 

 be under conditions essentially similar to plutonic magmas, and 

 may cool so slowly as to become holocrystalline. It is, more- 

 over, obvious that, could any superficial mass of erupted material 

 be traced back to its original deep-seated source, it would be 

 found to pass gradually from the volcanic to the plutonic type- 

 Hence it is that in the laboratory it is not always possible, from 

 the examination of the hand specimen or thin section only, to 

 determine to which of the two classes it may belong. We can 

 easily discriminate between the extremes, but there is a wide 

 intermediate zone where any such attempts are impracticable, as 

 indeed they are unnecessary.^ 



Owing to a false impression which formerly prevailed relative 

 to the nature of the Paleozoic effusives and those of Mesozoic, 

 Tertiary, and more recent times, dissimilar names have, in very 

 many instances, been applied to rocks which in other respects 

 than that of geological age are essentially one and the same. 

 Thus the name andesite is given to a rock in every respect 

 similar to porphyrite, with the possible exception of a slight 



^Whitman Gross has shown that there are exceptions to this rule. See 

 The Laccolitie Mountain Groups of Colorado, 14th Ann. Kep. U. S. Geol. 

 Survey, pp. 231-235. 



* Intermediate between these plutonic and effusive types is still a third 

 phase of prevailing holocrystalline porphyritie structure, thus far found only 

 in dikes, which it has been proposed to group under the head of d%ke roc/cs 

 (gangesteine). Since such are but local phases of plutonic magmas, which 

 have been left to cool and crystallize between narrow walls, instead of 

 poured out upon the surface, such. a subdivision seems scarcely called for 

 and as tending to still further confuse that which is already sadly con- 

 founded. The same may be said with reference to the now prevailing 

 tendency to give varietal names to every phase of magmatic differentiation, 

 and which has resulted already in such monstrosities of nomenclature as 

 ouaehitite, monchiquite, yogoite, and absarokite. 



