138 WARREN. 



single lamellae in these intergrowths is well under 1 mm., and usually 

 much under this figure, it is at once obvious that the total length of 

 measurement made in each case exceeds by many hundred times the 

 average length of a single lamellae.^ 



The presence of very small lamellae, often exceedingly minute, in 

 the microcline member, having their general direction parallel to that 

 of the larger ones, has been just referred to. In measuring each sec- 

 tion the practice was followed of omitting temporarily the measure- 

 ment of any lamellae that fell under two divisions of the scale used. 

 After the section had thus measured an attempt to estimate the 

 amount of these very small lamellae was made for each section. 

 This was done by using a Zeiss 3 mm. apochromatic objective with the 

 No. 12 compensating occular and a strong illumination. While only 

 approximate estimates were possible, it is believed that the figure 

 given in the table represents the true value for these lamellae to within 

 =1= 1.0 per cent.^ This estimate, approximate though it is, is certainly 

 better than a guess or than omitting them altogether from considera- 

 tion as was done by Makinen. 



In computing the weight percentages from those of volume obtained 

 directly from the micrometric measurements, 2.62 was used through- 

 out as the specific gravity of the albite member. This was the best 

 value found for the albite in two of the samples by direct determina- 

 tions with heavy solutions. For the microcline member slightly 

 different values were used in some cases, as shown in the column 

 headed Sp. Gr. In selecting these values we were guided by the speci- 

 fic gravity of the feldspar mixture as a whole. In the case of the 

 feldspar from Perth, Ontario, for example, the specific gravity of the 

 microclme is a little higher than the other because of the included 

 hematite scales. There is, of course, some chance for error here but 

 it cannot be as large as that introduced by the uncertainty regarding 

 the true amounts of the very small lamellae referred to above, for a 

 change of 0.02 in the specific gravity used would only effect a change 

 in the second decimal place of the weight percentages. 



7 No general rule for the length of measurements required for a desired 

 accuracy such as was laid down bj' Rosival (200 times the length of the diam- 

 eter of the average grain) can be adopted for measurements by this method in 

 thin-sections or in general. Tlie precision attainable must be estimated for 

 each separate case by the usual methods apphcable to such cases. 



8 The writer feels reasonably sure that the error is in all cases a negative one 

 for there are always a number of lamellae too small to be measured. The values 

 for the different sections were so nearly constant, that in view of the error 

 involved, it was deemed best to adopt for all what appeared to be the best 

 value, vis. 2.0%. 



