Pirsson — Phenocrysts of Intrusive Igneous Rocks. 275 



Moreover these flat phenocrysts, whether oriented or not, are 

 of great value in throwing light on another question in theo- 

 retic petrology — whether phenocrysts ever move in the magma 

 after it has come to rest ; that is, whether they sink down in 

 obedience to gravity. In the cases mentioned it is clear that 

 they have not moved or they would have arranged themselves 

 in obedience to the laws governing the movement downward 

 of flat plates through fluids ; they would be disposed with their 

 flat sides downward or perpendicular to the walls of the dike. 

 The writer has never seen such an occurrence nor been able to 

 find any description of one, and until such a case has been 

 found the evidence is against the idea that phenocrysts sink 

 through gravity and evidently because the magma is too viscous. 

 They could not assume such a position in an upward moving 

 magma because the motion must be differential and would arrange 

 them in the opposite direction ; hence this orientation would 

 show that they had moved and not the magma. The thicker 

 accumulation of phenocrysts at the bottom of an intruded 

 sheet, which has sometimes been noticed, would not prove they 

 had fallen through gravity unless they had this arrangement, 

 since that may merely show that the physical conditions neces- 

 sary for the production of phenocrysts were more favorable 

 and operative there than in the upper portion. 



To return again to the formation of phenocrysts in place, — 

 from the evidences given above, when we find that in dikes 

 and sheets flat tabular phenocrysts such as the feldspars of 

 many acid rocks or the large mica tables of certain basic ones 

 are scattered without orientation, we may conclude that they 

 have been formed in place and are not ''intratelluric." 



Evidence of porphyritic granites. — It is well known that 

 many granites contain large porphyritic feldspars, often of 

 great size. The granites may have so coarse a grain that in 

 the absence of these feldspars no one would think of them 

 as other than typical granular rocks. The same is also true at 

 times of other rocks besides granites. The evidence that 

 these afford is chiefly of a negative kind, but it is useful to 

 consider in this connection. It is evidence not so much of the 

 time and place of the formation of phenocrysts as of the cause 

 and manner of formation. To treat these questions fully 

 would lead us into the discussion of the " mise en place " and 

 the structure and classification of igneous rocks, which would 

 be beyond the limits of this article. It will therefore be suffi- 

 cient to ask one or two questions. Are these large phenocrysts 

 of granites to be considered intratelluric or not, or is granite 

 itself and all its constituents to be regarded as intratelluric ? 

 The difference between the porphyritic and the granular struc- 

 tures has been regarded by some, so far as their origin is con- 



