Pirsson — Phenocrysts of Intrusive Igneous Rocks. 277 



crysts have been rejected and pushed or not. In certain cases 

 the microlites of the ground mass have however an unequal 

 development — they may be thin flat plates like the feldspars of 

 some trachytes and andesites or slender, rods like the segirites 

 of some phonolitic rocks. In such cases it is common to 

 observe that the microlites have a length orientation parallel to 

 each face of the large crystal forming the phenocryst, and not 

 only this but the larger the phenocryst is, the more evident 

 this becomes ; they have been more crowded and the arrange- 

 ment is more evident. This applies of course only to those 

 microlites in the immediate neighborhood of the phenocryst. 

 In the case of the large feldspar phenocrysts of some Montana 

 tinguaites the crystals are covered with a perfect felt of simi- 

 larly oriented gegirite needles.* The arrangement is precisely 

 what would happen if a scattered group of matches lying on a 

 table should be swept to one side by a book and was caused 

 without doubt by a quite similar process. 



Such arrangements have been largely either overlooked or 

 probably confounded with fluidal or flow structures. The lat- 

 ter, caused by movements or currents in the crystallizing 

 magma, is however easily told, since in that case all of the con- 

 stituents of the period are alike affected, both phenocrysts and 

 microlites being drawn out into waving lines or streams, while 

 in case the orientation is produced by the expansion of the 

 phenocrysts the microlites are arranged parallel to each of its 

 crystallographic faces, have no orientation in the interspaces 

 and the phenocrysts themselves have also no orientation. This 

 expansion structure— -to coin a term for it — around phenocrysts 

 may of course occur in combination with fluidal structure and 

 merge into it, but typical cases of it are easily told and should 

 be more generally noted in petrographic descriptions. 



This evidence then goes to help the general proposition that 

 some phenocrysts are formed in the places where they now are, 

 for they were expanding and growing vigorously while the 

 groundmass was crystallizing and a magma whose groundmass 

 is crystallizing has already reached the place it is destined to 

 occupy. 



Summation of evidence. — The value of evidence is cumula- 

 tive and increases, not in an arithmetical, but in a geometrical 

 ratio. A single fact may indicate definitely a certain hypothesis 

 as its explanation, but it may also be an exception to a general 

 rule. When we can bring a second fact to support it, the 

 probability of the correctness of the hypothesis is greatly 

 increased, and when a considerable number of such facts are 

 found, all pointing to the same explanation, we may reasonably 

 consider it as proved. Thus, when we find that a dike con- 

 tains flat tabular phenocrysts, which show no orientation, and are 

 wanting in the border zone of contact and contain all the other 



*This Journal, IV, ii, 1896, p. 19]. 



