370 



A. C. LANE STUDIES OF GRAIN OF IGNEOUS INTRUSIVES 



meters, allowing for fractions not given there. There is, however, an 

 uncertainty of probably at least one-half a foot (152 millimeters) in most 

 of the distances. We obtain the table below : 



Section. 



Distance. 



Difference. 



Grain. 



Differ- 



Rate of dif- 

 ference. 



Ax. 



B. 











ence. 







15258 







296±152 







1.25 



.00422 







15257 



296±152 



7,054 " 



1.25 



1.65 



.00022 



.06 



1.19 



15256 



7350 '' 



8,550 " 



2.90 



1.1 



.00013 



1.58 



1.32 



15255 



15900 " 



6,200 " 



4. 



1. 



.00016 



3.42 



.58 



15254 



22100 " 



6,800 " 



5. 



2. 



.00029 



4.74 



.26 



15253 



28900 " 



8,600 " 



7. 



2.- 



.00023 



6.2 



.80 



15252 



36500 " 

 Lge from 1525 



/ to center . . 



9-6 







7.84 



1.16 



Avers 



.00021 





.885 













In this table the observations on thin-section 15258 are uncertain, for 

 it is decomposed, and those on 15252 and 15253 are not very satisfac- 

 tory, for the grain is so coarse that it would require a number of sections 

 to give us a fair idea. It is obvious, however, that from 15257 clear in 

 to the center, represented by 15252, the gradient is fairly steady and 

 somewhere about .0002, which is what we found for the group on page 

 129 of the Isle Royal report (.06/304). 



Taking the gradient from 15257 to the center, yve find it to be 

 (9-1.25) / (36,500-296) or .000213. Assuming this gradient A from the 

 margin, we find the column headed "^a;," and the difference between 

 these and observed values will give us the average value of B. These 

 differences are given in the column headed " ^." Then we can find the 

 contact zone /, which equals B j A, or 2,400 millimeters.^ This is so 

 much less than c, which is more than 73,000, that we see that we are 

 safe in using the approximate formulae, so far as that is concerned, and 

 c must be 2(36,500 -f 2400), or say 78,000 millimeters. 



As far as the ratio of temperature is concerned, if we use formula (19)t 

 we shall find that .45 hf^ will be about 2, and consequently /^ which is 

 equivalent to ujuo, will be greater than 1, which is impossible. In fact, 

 the formula for E does not hold when the grain increases to the center, 

 and uj Uo is too high, say about .90. We may therefore be reasonabl}'' 



* I believe this is really too high, and that part of the coarseness of grain at the margin, which 

 makes J? and hence?/ so large, is formed in flowing. I suspect y' should be about two-thirds of 

 the value used. 



fSee page 397. 



