492 PROCEEDINGS OF PHILADELPHIA MEETING. 



definitely measurable is evidently large) and to the natural fate of the fragments 

 which are to be cast upon the sea beaches and from them come to the deposits which 

 are forming in the littoral zone or to be dissolved in the sea water. Tlie farther 

 consideration may be noted, namely, that in the region where volcanoes commit 

 large amounts of finely divided lavas to the sea, with a consequent speedy forma- 

 tion of sediments, the result is likely to be a more than usually rapid upward move- 

 ment of the isogeothermal planes in rocks containing a large proportion of water. 

 This would naturally lead to explosions of exceeding violence, such as characterize 

 the Javanese archipelago. 



The paper by President Shaler was discussed by C. H. Hitchcock, C. 

 W. Hayes, L. V. Pirsson, M. E. Wadsworth, Persifor Frazer, W. M. Davis 

 and the author. 



The following paper was read b}^ the autlior : 



A NEEDED TERM IN PETROGRAPHY 

 15 Y L. V. PIRSSON 



\_A bstraci] 



The term crystal when strictly and i)r(3j)erly applied means the geometrical form 

 assumed by the physical molecules of a definite chemical compound or by isomor- 

 phous compounds in crystallizing — that is, in arranging themselves according to 

 certain laws of symmetry. Thus on hearing the term crystal we always imagine to 

 ourselves this outward symmetrical form. 



The demands of petrograi)hy have, however, often conferred n])on the term an- 

 other sliglitly ditterent concei)tion, in which greater stress is laid upon the internal 

 molecular structure and physical properties of the body than upon its outward form. 

 Thus we hear of " idiomori)hic" crystals, though using the term in its strict sense 

 every crystal nuist be idiomorphic. 



This being the case, it seems clear that we have no good term to exi)ress those 

 bodies which, though possessing the internal molecular structure and physical 

 pi'oi)erties of crystals, have through the conditions of their growth not been able to 

 attain outward symmetry. 



Thus in augitic rocks we often find, on the one hand, cases where the pyroxene 

 has crystallized freely as a phenocryst ; it is idiomorphic, possesses its outward form 

 bounded by crVstal faces and is truly a crystal. On the other hand, we frequently 

 find cases where the pyroxene has had its growth interfered with by that of other 

 minerals and it has no definite geometrical form ; it may often be found in gran- 

 ules or in rounded or ellipsoidal bodies. 



While these latter forms are generally called cr3''stals, referring especiall}' to their 

 internal structure, a survey of the literature will show that mineralogists, and espe- 

 cially petrographers, have felt the need of a more precise and definite term to de- 

 nominate them. Thus we sometimes find them called " crystal fragments," a usage 

 which is objectionable, since, if literally taken, it would mean that they were por- 

 tions of a formerly larger mass of the same material, which they by no means are. 

 Again, others have called them "crystalloids," but a difiiculty in the use of this 

 term lies in the fact that it has long had a perfectly definite and well defined 



