376 



A. C. LANE — STUDIES OF GRAIN OF IGNEOUS INTRUSIVES 



A second set of specimens were collected by Mr F. E. Wright at an- 

 other point not far off, where the dike is about 15,200 millimeters wide, 

 and the contact with the Mona schist is 20 paces east of the fence at the 

 foot of Circle street, Marquette. We had one section which was made 

 5 centimeters long, to cover the 50 millimeters next to the contact. This 

 shows that even at the margin the increase of grain is not so much 

 greater as it would have to be if the slope we have called Cwere A. 



The results of observations at different distances are given below. It 

 must be remembered that when the grain is very fine it is difficult to get 

 an accurate idea of it. The average grain for the three center slides 

 agrees well with what we found before. Eis 1.07. Up to 670 millimeters 

 the rate of increase is .00046, which is quite as near to that pre- 

 viously found as we could expect, but the grain of the augite in number 

 4, at 1,890 millimeters, is certainly much finer, and that at number 6, 

 at 4,115, perhaps somewhat coarser than normal. I have made repeated 

 sets of observations on number 4, and can only account for the fineness 

 as due to some initial irregularity of temperature or composition. 



Distance. 



Difference. 



Grain. 



Difference. 



S. 





' 

















10 





.012 



.0012 





10 





.012 











10 





.005 



.0005 





20 





.017 







- 



30 



10 



.023 



.006 



.0006 







10 





.009 



.0009 





40 





.032 











10 





.010 



.0010 





50 





/ .042 

 \ .04* 









400 



.123 



.00031 



456 





.165 









166 





.145 



.00087 



670 





.310 









1,274 



1 .43 to 



.12 



.00009 



1,890 





.59* 









2,225 



i .444* 



.83 



.00039 



4,115 





1.26 

 1.00* 









1,645 



—.52 





5,760 





r .78 



I .93* 









1,840 



.39 





7,600 





(1.17 

 {l.l* 















Medford Dike 

 The grain of the Medford dike is described in discussing the porphyritic 



* Figures obtained upon revision, the second set of five sections, not quite so carefully meas- 

 ured. As there is none of the country rock visible in section 1, a few millimeters should perhaps 

 be added to the distance. A further discussion of this dike will be left till later. 



