WARKEN. — ALKALI-GRANITES AND PORPHYRIES. 313 



into the grain of the rest of the rock, which is hke that of the coarse- 

 granite but much finer. We have here probably to deal with a sud- 

 den, initial chilling of the magma for a certain distance from the slate 

 contacts, followed, however, by a relatively slow dissipation of the 

 heat when compared with that of the quickly cooled magma at higher 

 levels, which developed the granite-porphyry. We may assume that 

 the sudden, initial chilling caused a sufficient increase in the \'iscosity 

 to determine more numerous centers of crystallization, while the rela- 

 tively slow cooling that ensued under the thicker cover of slate per- 

 mitted the development of an essentially even grained though fine 

 texture. The conditions, like the results, were intermediate between 

 those of the granite-porphyry on the one hand and the coarse-granite 

 on the other. It is also conceivalile that the magmatic water and 

 other materials capable of aiding crystallization were retained better 

 in the deeper seated magma than they were in the porphyry-magma, 

 located as it was much nearer the surface. 



Differcutiation of the Rhombcii porphyry. — We may next consider 

 the crystallization of the rhonibenporphyry and its bearing on the dif- 

 ferentiation of this rock. Here there is no question but that the rhom- 

 benfeldspar phenocrysts — homogeneous mixed crystals as we hold 

 them to be — were the first minerals to crystallize. Before the close of 

 *the growth of these feldspar phenocrysts the calcic-iron pyroxene l)egan 

 to crystallize. Again we note a pause, marking the close of the pheno- 

 crystalline stage of growth, followed by the consolidation of the 

 groundmass about more numerous centers, with a practically simul- 

 taneous growth of the minerals and with the addition of a part of the 

 groundmass feldspar — now cryptoperthite or microperthite — to the 

 feldspar crystals already formed. In such a texture there is, in the 

 writer's opinion, proof that the rock was not formed by any process of 

 fractional crystallization. It indicates that the rock was crystallized 

 from an individualized — dift'erentiated — ^ magma of homogeneous com- 

 position, at least through such parts as now possess a uniform texture 

 and composition. It is true that the rhombenporphyry as a whole 

 does not possess a perfectly homogeneous composition, and as sodic- 

 pyroxenes and hornblende enter into its composition in some parts, 

 we have doubtless a gradation toward the granite-porphyry. The 

 composition of a part of the xenoliths, obviously fragments broken 

 off from the main masses of this porphyry, show a somewhat further 

 gradation toward the granite-porphyry. The actual contacts between 

 the rhombenporphyry and the other rocks are always sharp, and if 

 there ever was a complete transition between them it is necessary to 



