Oil two new Varieties of Vermiculiles. 135 



one of the prisms ; for the conditions already stated that decide 

 the diameters of the prisms, a b, fig. 4, equal to the distance be- 

 tween their centres A, B, decide also the length of line a I, 

 in which the contractile strain becomes sufficient to fracture the 

 material. The distance a I will actually somewhat exceed this, 

 because the temperature of the prism at the place of the first 

 joint is higher than at the surface S S ; and the distance between 

 the first and the second joint lower down will be somewhat greater 

 than that of the first from the top ; but after a certain length 

 has been attained by the prisms, if their material were perfectly 

 homogeneous and isotropic and the temperature of the unsplit 

 mass the same at all depths, the distance between the joints 

 would become constant, and never greatly exceed the diameter 

 of the prism if that be large. 



In nature the mass of basalt is never perfectly homogeneous 

 or isotropic ; and we shall hereafter consider the effects of the 

 want of homogeneity, as well as of the diameter of the prisms, 

 in disturbing the lengths between the joints. 

 [To be continued.] 



XVII. On two new Varieties of Vermiculites, with a revision of 

 the other members of this Group, By Josiah P. Cooke, Jun., 

 Erviny Professor of Chemistry and Mineralogy at Harvard Col- 

 lege, and F. A. Gooch, Assistant in the Chemical Laboratory*, 



SINCE the publication of the writer's first monograph on the 

 vermiculites -f, two new varieties of this group of minerals 

 have been brought to his notice by Mr. W. W. Jefferis, of West 

 Chester, Pa., who has most kindly furnished the materials for 

 the following investigation. 



The first of these varieties (which occurs at Lerni, Delaware 

 Co., Pa.) has the following characters : — The unaltered mineral 

 is of a dull sea-green colour, has a highly developed micaceous 

 structure, is an aggregate of rough hexagonal plates, and of very 

 imperfect external form. It is transparent in moderately thin 

 lamina?, and is free from enclosed foreign matter. The optical 

 characters of the mineral closely resemble those of the Culsagee 

 variety of vermiculite, the angle between the optical axes varying 

 in different parts of the same laminse from 18° to 0°. Its hard- 

 ness is about 1*5 ; and three determinations of its specific gravity 

 (taken in alcohol at 23° C.) gave 2'409, 2'368, and 2'373. 



* From the Proceedings of the American Academy of Arts and Sciences, 

 presented May 11, 1875. Communicated by the Author. 



t " The Vermiculites, their Crystallographic and Chemical Relations to 

 the Micas," by Josiah P. Cooke, Jun., Phil. Mag. vol. xlvii. p. 241. The 

 analytical work in this second paper has been done by Mr. Gooch. 



