274: Pirsson — Phenocrysts of Intrusive Igneous Pocks. 



Evidence of dikes and sheets. — In many cases we have clear 

 and indisputable evidence, that systems of dikes and sheets 

 have been formed by a single geological act ; that is, filled 

 from a single parent magma, at the same time and under the 

 same conditions, and they have of course the same chemical 

 composition. An example occurring in the Judith Mts.* of 

 Montana has been recently described by the writer. The 

 attendant fringing sheets lying above many laccoliths may 

 also be cited as examples. Yet in many of these cases some 

 sheets or dikes may have large and well-developed pheno- 

 crysts, others none at all. The laccolith may be fall of pheno- 

 crysts ; its attendant sheets or some of them may be entirely 

 devoid of them. A sheet of tinguaite in the Judith Mts. con- 

 tains phenocrysts of feldspar as large as one's hand, but a nar- 

 row sheet of the same rock which the general geologic history 

 of the district shows was intruded at the same time, and which 

 has a similar composition, has only very minute ones arranged 

 in flow structure. In these cases, had they previously existed 

 in the parent magma, they should be found in both alike. 

 The conclusion in these cases is that the phenocrysts have 

 formed in the places where they now are. 



Evidence of fluidal phenomena. — In the case of some dikes 

 and sheets it has been noticed that the phenocrysts of feldspar, 

 which have a flat tabular form, have a definite arrangement 

 throughout the mass, with the flat sides parallel to the bound- 

 ing walls, or in other words, as it is commonly called, a 

 "fluidal" or "flow structure." Sometimes they are rather 

 irregularly distributed, collected in groups or in lines, and are 

 just as apt to occur close to the border zone, at the bounding 

 wall, as elsewhere in the dike. Such instances show clearly 

 the effects of tabular plates being moved along in a liquid, 

 and they prove that the phenocrysts were formed in the 

 magma before it came to rest. How long before we are of 

 course unable to determine, but in these cases they may be 

 u intratelluric," so far as any definite meaning can be attached 

 to this term. 



In other dikes and sheets, the phenocrysts have no oriented 

 arrangement in the mass, their orientation is wholly hap- 

 hazard, they occur evenly distributed throughout, or they may 

 be very small or of different character, or even wholly want- 

 ing in the border zones, and this in spite of the fact that they 

 may have a pronounced tabular development. 



If in the former cases mentioned the evidence proves that the 

 phenocrysts were formed prior to injection, then the absence of 

 such evidence, that is to say its converse, proves just as clearly 

 that these flat phenocrysts were formed after injection when 

 the magma had come to' rest or in other words in place. 



* 1 8th Ann. Rep. U. S. Geol. Surv., pp. 551 and 572, 1898. 



