CONCEIVABLE CLASSES OF PHEXOCRYSTS 387 



though they may afterward be swept into the center by the current. It 

 is also possible that the crystallization of one mineral ma}^ proceed 

 throughout the magma and motion take place afterward. Cases of this 

 I refer to later. 



3. If the conditions of consolidation of a mineral are close to those 

 which the magma had when it reached its destination, its grain will in- 

 crease continuously to the center, while later formed minerals will have 

 a narrower zone of increasing grain. In such a case the mineral will be 

 more prominent at the center than at the margin, must be among the 

 earliest of the last generation, and indeed may very likely have begun 

 formation during the eruptive act, and so have a core which belongs to 

 class 2. 



4. If the conditions of consolidation of a mineral are nearly half way 

 between those of the magma and the country rock into which it is in- 

 jected, then the physical condition of the margin will be nearly that of 

 slowest cooling, and very large crystals may be formed there. These 

 crystals may be earlier than some, later than others in order of formation^ 

 but I think usually late. 



Theoretically, if the temperature were just half way, the grain at the 

 margin, supposing the mineral to be formed at that temperature, would 

 be infinitel}^ greater than that at the center. In plate 58 the curve 

 marked .52 shows the variation of grain from center to margin, su2)pos- 

 posing the temperature of formation of the mineral were .52 of the ini- 

 itial temperature of the magna, that of the country rock being taken as 

 0°. Practicall}^, however, no mineral is formed exactly at any one 

 temperature, and the real effect should be that the grain near the margin 

 should be under these conditions very irregular for a narrow belt and 

 exceedingly sensitive to slight variation, ranging from great coarseness 

 to great fineness, just as we see in plate 58 that for a fifth of the way in 

 it makes a big difference Avhether the initial temperature is such that 

 the temperature of consolidation is .40 or .60 or .52, but thence in toward 

 the center it makes much less difference. 



5. Finalh^, there are crystals, notably of staurolite and chloritoid (but 

 garnet, biotite, hornblende, epidote, and other minerals also occur sim- 

 ilarly), which are extra large, but are produced by later slow metamor- 

 phic action in sediments as well as igneous rocks. They are often 

 honeycombed, canaliculated with cavities and enclosures of the other 

 constituents. 



These five kinds may be distinguished as food, floating, early, border, 

 and metamorphic crystals or phenocrysts. If the distinction of these 

 classes is important enough to call for uniform names in all tongues, 



