MEDFORD DIKE 



401 



which I have studied, and has been described by many writers. Their 

 writings are cited in a recent monograph by A. W. G. Wilson, issued by 

 the Boston Society of Natural History, so that I will proceed at once to 

 some observations which I made on the grain at Powder House hill. 

 Unfortunatel}^ I was unable to find the exact contact, but the first sec- 

 tion taken was close to it, judging from appearances. Below we give 

 the results of observations : 



No 



Distance from I in millimeters 

 Grain of augite in millimeters 



Macroscopic 



Microscopic 



.83 

 2.05 



II HI IV 



560 



1.36 

 2.15 



1615 



4.23 

 4.05 



2240 



3.13 

 1.65 



V 



10000 



1.92 

 1.04 



YI VII VIII 



20000 



1.74 

 1.21 



30000 



1.62 

 1.45 



40000 



1.69 

 1.17 



The macroscopic observations were made on the hand specimens to 

 get a general idea of the grain, and do not pretend to be accurate. The 

 microscopic observations are derived from the average of the dimen- 

 sions at right angles of five of the larger augite grains taken in each 

 slide. The grain of specimens II and III was subjected to special 

 verification. Three extra observations on each specimen gave for the 

 grain of the augite 2.47 and 3.86 millimeters respectively, so that there 

 can be no doubt as to the existence of a belt of coarse augite near the 

 margin.* The grain will therefore obviously have a curve of the type 

 of .52, plate 58. The marginal grain slope C is hardly less than .0017 

 and the margin of the dike can hardly be more than a meter from 

 I and is most likely considerably less. The grain in V, V^I, VII. VIII 

 is obviously practically the same, so that we have the average grain of 

 the coarse central belt of uniform grain 1.32 millimeters. Now, whether 

 we start from the fact that the zone of uniform grain is not less than two- 

 thirds the breadth of the dike, after which there is an increase, or from 

 the fact that the augite is as great as at the center within a meter or so 

 from the margin, we shall arrive at a value of uluo^ but little above .50. 



The temperature of consolidation of the augite is therefore just a little 

 nearer that of the injected magma than that of the country rock. In 

 this case the augite is practically the last formed mineral and the others 

 occur imbedded in it. 



Marginal Phenocrysts 



Suppose, however, the above case that the augite, instead of being 

 among the latest formed minerals, was among the earlier formed. It 



*Mr L. C. Graton told me that near Roberts station, north of Georgian bay, he had seen dikes 

 with a coarse grained seam near the margin. 



LVI— BuLi,. Geol. Soc. Am., Vol. 14, 19n2 



