1820.] Mr. BrookeonMesotj/pe,Needlesto)ie,andThomson!te. 193 



Article V. 



On Mesotype, Needlestoiie, and Thomsonite. 

 By H. J. Brooke, Esq. F.R.S. 



(To Dr. Thomson.) 



SIR Keppel-street, Aug. 14, 1820. 



In the several published works on mineralogy, the substance 

 denominated mesotype is said to be found in Auvergne, in Ice- 

 land, Ferro, near Dumbarton, 8cc. and a square prism is given by 

 the Abbe Haliy as its primary form. This form, however, does 

 not belong to any specimens I have seen from those localities, 

 nor do the substances themselves belong to the same species. 



In the following notice I shall call the Auvergne variety, 

 Mesoti/pe; that from Iceland and Ferro, Need/estone ; and that 

 from Dumbarton, Thomsonite, after the editor of this journal, who 

 has contributed so largely to the improvement of chemical 

 analysis. 



The specific gravity of the mesotype is 2*24 



needlestone 2*27 



thomsonite 2"37 



Mesotype, from Auvergne. 



Among the first specimens I examined of this substance, I 

 observed the summits of some of the crystals to consist of eight 

 planes, as in (PI. CVII) fig. 8, four of which, dd',ff, were 

 incompatible with the supposition of a square prism being the 

 primary form. And on submitting to the reflective goniometer 

 the planes obtained by cleaving the crystals parallel to the 

 natural planes, M, M', of the prism, I found the inclination of 

 those planes which aftbrded the best reflections to be 91° 10'. 

 The inchnations of the terminal planes c, c', e, e' , on the sides of 

 the prism were also all equal, the primary form is, therefore, a 

 right rhombic prism; and if the planes c, c', e, e' , result from a 

 decrement by one row on the terminal edges of the primary 

 crystal, the height of the prism will be to its terminal edge very 

 nearly as one to two. The planes d, d' , are the result of an 

 intermediary decrement on the acute angles of the prism. 



The measurement of M on c is 116° 37' 



c e' 126 47 

 c d 178 45 

 c c' 143 14 

 d d' 145 44 



Needlestone from Ferro. 



Besides the difference in specific gravity, the needlestone 

 differs from the mesotype in some other of its characters. 

 Vol. XVI. N° III. N 



