OF ARTS AND SCIENCES. 51 



parallel to this diagonal, and, like the specimens from "West Chester, are 

 frequently jointed in this direction. Like other micaceous minerals, the 

 plates cleave readily parallel to the basal plane, yielding very thin foliae, 

 exceedingly flexible, but not elastic. The optical characters are the same 

 as those of theAVest Chester variety, — strong negative double refraction 

 yielding a biaxial ring system, with uniform distribution of colors, and 

 very variable optical angle. I have measured angles from about 30° to 

 about 13°. The angle often varies widely in different parts of the same 

 plate. Thus I have measured on different laminte, from a single plate 

 not exceeding 3 inches in diameter, the three angles 30"^, 24°, and 13"; 

 and again I have noticed a similar variation on one and the same 

 lamina. Indeed, the phenomena which I observed were almost identical 

 with those I had previously observed on plates of ripidolite from Texas, 

 Pennsylvania, he. cit. On moving the lamina just referred to parallel to 

 itself, in front of the polarizing microscope, the optical angle varied as I 

 passed from one side of the field to the other. Beginning with a value 

 of about 30'^, the angle decreased to about 13'^. Moving the plate 

 still further, I found a region of indistinctness, and then the axes opened 

 again, — the new plane making an angle of 120° to the old. I had 

 evidently here a macling precisely similar to that 

 I had previously described in the Texas ripidolite, 

 and shown in Fig. 3, where the lines of shading 

 represent the position of the plane of the optical 

 axes. This represents what we may call an ideal 

 made ; for I have seldom been able to trace more 

 than three individuals on the same plate, and, as a 

 rule, these are very unequally developed. On many 

 of the specimens of the North Carolina vermiculite, the macling is exter- 

 nally marked by the eminent cleavage or jointing parallel to the shorter 

 diagonal of the rhomb section, and in several of the specimens I have 

 examined it was quite symmetrical. A study of these specimens led me 

 to an explanation of the cause of the remarkable variation of the optical 

 angle, which I believe not only applies to the vermiculites and ripido- 

 lites, but also, in many cases at least, to the micas. It would be expected 

 that the several members of such macles as Fig. 3 represents would 

 peneti'ate each other, and I therefore made a series of experiments to 

 ascertain what would be the effect of the interfoliation of laminae in 

 which the planes of the two sets of optical axes had the same relative 

 position as in the several members of the made. To that end I divided 

 a plate, which presented the largest optical angle I had observed, into 

 as thin laminas as possible, and then superimposed them in the relative 



