142 



SCIENCE. 



[N. S. Vol. IX. No. 213. 



On the Phenocrysts of Intrusive Igneous Rocks. 



L. V. PiEssoN, New Haven, Conn. 



The speaker argued for the formation of 

 phenoprj'sts at or near tbe places where 

 they are found in rocks, and against the 

 necessity of the generally accepted idea 

 that they are deep-seated and older crystal- 

 lizations brought up by the magma, i. e., 

 against the necessary ' intratelluric ' nature 

 of them. He distinguished the 'single' 

 type which does not occur as a mineral of 

 the ground mass and the ' recurrent ' type 

 which does. As incompatible with an in- 

 tratelluric origin, he advanced the follow- 

 ing well-known phenomena : (1) Absence 

 of phenocrysts from contact zones. (2) Ab- 

 sence from dikes and sheets whose parent 

 laccolite is richly provided with them. (3) 

 The throngs of small rod-like crystals that 

 surround phenocrysts and are not flow- 

 phenomena, but due to crowding back, by 

 growth of phenocrysts ; further tabular 

 phenocrysts which occur in all orienta- 

 tions in a rock. (4) Phenocrysts of pro- 

 phyritic granites may or may not be intra- 

 telluric, according as we view porphy- 

 ritic rocks as differing from granitoid in 

 kind or in degree. (5) Micro-structure, 

 both internal as regards inclusions and ex- 

 ternal as regai-ds surrounding crystals, may 

 be explained by formation near the surface. 

 The arguments for an intratelluric origin, 

 viz.: (1) Large size, and (2) flow-arrange- 

 ment and resorption-phenomena, were dis- 

 cussed. As opposed to the views of the 

 French petrographers, that there are two 

 distinct periods in the crystallization of 

 every igneous rock, and of the Germans, 

 that there are two for the porphyritic and 

 one for the granitoid, Pirsson argued for 

 only one for each, and emphasized the vis- 

 cosity of the magma as an important fiictor 

 in conditioning the epoch of crystallization, 

 and the rate of cooling as of great influence 

 on the result. With a long time, i. e., slow 

 cooling, the granitoid texture results; with 



a short period, the porphyritic or felsitic. 

 The presence of water-vapor is also impor- 

 tant. With a quick fall in temperature the 

 earliest minerals to begin have the best 

 chance to develop; the later ones are hurried 

 or are cut off. Hence, single phenocrysts 

 result. Mass action is also important. The 

 most abundant minerals have a predominant 

 tendency to develop. Too great regularity 

 is not, however, to be expected in Nature. 

 The speaker closed with a statement that 

 he had no hopes of the Section agreeing 

 with him, but he courted discussion. He 

 was mildly thunder-struck to find very 

 general agreement and approval as evinced 

 in remarks by J. P. Iddings and Whitman 

 Cross, although, the hour being late, the 

 paper could not receive the attention that 

 its importance and interest merited. 



The last paper of the Section was the 

 following : 



The Mica Deposits of the United States. J. 



A. Holmes, Chapel Hill, N. C. 



The speaker stated that to-day all the 

 commercial mica produced in the United 

 States is derived from North Carolina. It 

 is universally obtained from pegmatite 

 dikes, in which as a maximum not or over 

 1 or li per cent, of the dike is mica, and 

 about 0.1 per cent, is the rule. About 5 per 

 cent, of this mica or less is merchantable as 

 sheets; the rest, if utilized, is ground. The 

 chief defects are the crushing and warping 

 due to dynamic processes, and the so-called 

 ' ruling ' or cleavage which runs across the 

 leaves and is probably due to pressure. 

 The speaker described in particular the 

 mica deposits of New Mexico, where the 

 pegmatites are associated with granites at 

 the base of the Grand Caiion series and are 

 older than the Algonkian. They are dam- 

 aged by folding and pressure, which, how- 

 ever, largely fail in the Appalachian belt. 

 The hour being late, no discussion followed, 

 and after a vote of thanks to the authori- 



