300 TJie American Geologist. May, i9oo 
ety, Professor L. V. Pirsson read his paper "On the pheno- 
crysts of intrusive igneous rocks,"* my first impression was 
that his conchisions were practically identical with my own; 
but a careful reading of his paper after its publication con- 
vinced me that they are sufBciently distinct to warrant this ad- 
ditional contribution to the literature of the subject. We 
agree in our dissatisfaction with the long-accepted theory that 
an abrupt change, or any appreciable change, in the rate of 
cooling is essential to the differentiation of phenocrysts and 
groundmass, or the development of the porphyritic texture in 
plutonic and intrusive as well as in effusive rocks; and also 
in regarding the viscosity of the magma as an important fac- 
tor; but beyond this the development of our ideas is along 
quite different lines. 
Prof. Pirsson first points out the utter failure of the gen- 
erally accepted theory to explain some of the most important 
facts, such as: the more frequent occurrence of phenocrysts 
in the central than in the peripheral portions or contact zones 
of dikes ; the development of the porphyritic texture in a part 
only of a continuous, contemporaneous and homogeneous sys- 
tem of dikes or sheets ; the relations of phenocrysts to flviidal 
phenomena; the coarsely porphyritic granites, a sudden cool- 
ing being inconceivable with this deep-seated, plutonic mag- 
ma; the inclusion, and also the crowding back, of crystals "of 
the groundmass by the phenocrysts, indicating the subsequent 
development of the latter. He then develops his own view, 
according to which the most important factor in the crystalli- 
zation of rocks is the crystallization interval or the period dur- 
ing which the crystals of a particular mineral continue to grow 
before the increasing viscosity of the magma determines the 
establishment of new centers of crystallization. With long 
crystallization intervals each constituent may exhaust itself in 
the magma before the next begins to crystallize. When each 
has had its turn, the rock has crystallized, and the granular 
structure is formed. A'iscosity, which varies inversely as both 
temperature and hydration, determines, per se, simply more 
numerous centers of crystallization for each mineral and a 
finer grain of the resulting rock. But increasing viscosity dur- 
*Am. Jour. Sci., 157, 271-280. 
