1868.] On the Optic Axes in Harmoiome and Wdhlerite. 319 



water lakes, extending from Galilee to the Nyanza, Nyassa, and Zambesi, 

 when an ichthyological fauna was developed suited to the warm conditions 

 that prevailed, part of which survives in the Jordan. 



During the glacial period the temperature of Lebanon must have been 

 similar to the present Alps, as the existing mammals and birds on the 

 summits are identical with those of the Pyrenees and the Alps ; not so the 

 glacial flora, of which almost every trace has been lost. But the flora had 

 not the same powers of vertical migration with the fauna, of which, how- 

 ever, the Elk, Red Deer, and Keindeer, found in the bone-caverns, have 

 long since perished. 



During the present period the Mediterranean forms have overspread the 

 whole country, excepting the mountain-tops at an elevation of 9000 feet, 

 and the Jordan depression. These two exceptions can be best explained 

 by the fact that the traces of the glacial inroad are not yet wholly oblite- 

 rated, and that the preceding warm period has left its yet stronger mark in 

 the unique tropical ''outlier" of the Dead-Sea basin, analogous to the 

 boreal outliers of our mountain-tops, the concave depression in the one 

 being the complement of the convex elevation in the other. 



II. New Researches on the Dispersion of the Optic Axes in Harmo- 

 tome and Wohlerite^ proving these Minerals to belong to the 

 Clinorhombic (Obhque) System.'' By M. A. L. 0. Des 

 Cloiseaux. Communicated by Prof. W. H. Miller, For. Sec. 

 R.S. Received March 12, 1868. 



(Abstract.) 



We are already acquainted with a considerable number of crystals, 

 natural as well as artificial, the forms of which have only been determined 

 with precision by the examination of their optical properties as doubly 

 refracting bodies. Harmotome and Wohlerite furnish two fresh examples 

 of this ; and they afford all the more important proof of the necessity of 

 appealing to these properties^ inasmuch as the crystals of these substances 

 would appear certainly to be derived from a right rhombic prism, so long 

 as we consider only the apparent symmetry of their external forms, or the 

 orientation of the plane containing their optic axes. The different sorts of 

 dispersion which these axes might be capable of presenting are so feeble, 

 and so difficult of appreciation on account of the slight transparency of 

 Wohlerite, and the complex structure of the crystals of Ilarmotome, that 

 the determination of these dispersions has hitherto been too incomplete to 

 allow of any conclusion being drawn as to the crystaUine type they might 

 otherwise serve to characterize. 



It was a remark of M. Axel Gadolin that induced the author to resume 

 the attentive study of the phenomena of dispersion, first in Harmotome, 

 and then in Wohlerite, and as a consequence to modify the crystallographic 

 type to which these minerals have been in general referred. 



