FREQUENCY OF UNEXPECTED DISCOVERY OF IGNEOUS ROCKS 233 



much which has puzzled our contemporaries and our predecessors. They 

 begin when an uprising molten mass stops and crystallizes ; they cease 

 only when its emissions and its stores of excess heat as compared with its 

 wall-rocks are exhausted. 



Frequency of the unexpected Discovery of igneous Eocks 



At the outset let me remind you that we are never safe from igneous 

 rocks. No matter how undisturbed sedimentary strata may appear, the 

 geologist would be rash who would dismiss the possibility of a dike or 

 neck, or even of a deep-seated mass. A few illustrations will suffice. 

 Among the older strata one might think of the almost undisturbed Paleo- 

 zoic formations of central New York as most unlikely scenes of igneous 

 action, but from accumulated observations extending over nearly a cen- 

 tury we now know of the alnoite and mica-peridotite dikes which begin 

 in the faulted Trenton at Manheim Bridge, 1 near Utica; appear in the 

 ►Salina at Geddes and Syracuse; 2 pierce the Devonian north and south of 

 Ithaca ; 3 cut the Monongahela series of the Coal Measures in the Connels- 

 ville basin of western Pennsylvania; 4 and outcrop at intervals across 

 Kentuckv 5 and Arkansas 6 in strata of the reverse and descending order 

 of age. While the dikes may not all belong to the same time of outbreak, 

 there is a striking similarity in their mineralogy. 



Now, even a small dike implies a much larger reservoir in depth. It 

 follows that these intrusives, so closely similar in petrographic character, 

 are in all probability the most basic and most fluid differentiates of a 

 vast and widely distributed series of related magmas. If not all, at least 

 one of the magmas forced its way upward at a time appreciably later than 

 the age of the latest strata cut — that is, than the Monongahela series of 

 the Pennsylvanian. How much later we have no direct means of know- 

 ing, but in the Connelsville coal basin the dike has extensively coked the 

 Pittsburgh seam. If we are justified in the inference that this seam was 

 coking bituminous coal when the dike entered, then we have to allow a 

 long time to have elapsed before the original peat could have reached this 

 stage. 



There is not very much, if any, good coking coal the world over of Ter- 

 tiary age, in gently folded strata, uninfluenced by intrusive rocks. We 

 seem to need a longer period of time than the Tertiary to bring about 

 these changes; and, on the assumption that the Pittsburgh seam was 

 coking coal when the dike came in, we are forced to place the time of 

 intrusion at least one long geological period after the close of the Penn- 

 sylvanian. It may have been several. We may shorten the assumed time 



