1910.] 



NATURAL SCIENCES OF PHILADELPHIA. 



33 



6o 61 6V 66 63 70 7* 7 u 7 s -jg 



Fig. 9 — Axial index — 170 Michigan, major form. 



The axial index of the major form varies from .59 to .78, with an 

 average of .67. 



The curve is multimodal, culminating at .65, with minor modes at 

 .63,*.67 and .71. 



The similarity between this curve and that for the entire Michigan 

 series is very striking, and both have a general resemblance with that 

 of the Cincinnati series, but show greater variability. 



•Tf 61 6i1 06 61 io 72 14 TH-ms. 



Fig. 10— Axial index — 55 Michigan, minor form. 



The index of the minor form ranges from .59 to .74, with an average 

 of .66. The curve is also multimodal, but shows greater variability 

 than in the typical series. The major mode is at .67, with minor 

 modes at .61, .63 and .71. 



The Isle Royale Series. 



This series is of interest as being, like that from Cincinnati, repre- 

 sentative of a comparatively restricted region with no great variation 

 3 



