374 



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



We are therefore justified in assuming that s is equal to C — that is, 

 .5570/23210= .000024. This value for C will make Cx' satisfy all the 

 observations with errors well within the limits of error of observation. 

 If the feldspar is formed before the augite, uj u^ must be greater than 

 .78 and the grain at the center must be between s .2io — that is, 5.8 mil- 

 limeters and a value to be found from equation (15), of page 396, or 2.66 

 millimeters. Now, Kiimmell says that at the center the thin tabular 

 cr^^stals are f to f inch in diameter, but judging from Queneau's photo- 

 graphs of the general habit of feldspar in diabases we ma}^ safely assume 

 that the thinness is about \ of the breadth. This should make the mean 

 dimension of sections at right angles to the pinacoid M from 3 to 4.5 

 millimeters in linear dimension — ^.that is, in area 9 to 2O4 square milli- 

 meters, so that there is a fair accordance. 



Diabase Dike at Light House Point, Marquette 



The grain of the Light House Point dike was somewhat discussed by 

 me in the Isle Royal report,^ but I made the mistake of not considering 

 the ambiguity as to whether the gradient on increase was Ax' -\- B ox 

 Cx'. The average observations on a set of sections more recently taken, 

 July 17, 1902, are (accurac}^ of measurements not over 10 per cent) — 



Distance. 



DiflFerence. 



Grain in 

 millimeters. 



Difference. 



Ratio s. 



Ax\ 









.046 











50 





—.028 



—.00056 





50 





.018 







.0275 





125 





.061 



.000485 





175 





.079 







.0965 





231 





.137 



.000595 





406 





.216 







.223 





355 





.194 



.000518 





761 





.410 







.419 





861 





.259 



.000303 





1622 





.669 











6608 





.344 



.000052 





8230 





1.013 









The specimens at 8230 is from the center of the dike, so that 2w is 

 16460 millimeters. The specimen at 1622 millimeters was taken at 

 what seemed to be the end of the zone of increasing grain, and it is 

 obvious that if the rate of increase which holds from the margin up to 

 761 millimeters continued the grain of the augite would there be .9 mil- 

 limeters, or nearly the same as at the center, so that the field estimate 

 was not very far out of the way. 



Pp. 243 to 245. 



