408 Dr. Thomson's Aimli/sis of Needlesfone. [De^, 



Mesolite of Fuchs 3 A S + (| S + ^ C) S' + 2.«- Aq. 



Mesolite of Freyssmuth. . . 3 A S + (a. S + a. C) S^ + 3 Aq. 



In the Annals of PhilosopJn/ for September, 1820, (vol. xvi. 

 p. 193,) Mr. Brooke, so well known for his profound knowledge 

 of crystallography, and the enthusiasm with which he has 

 devoted himself to the study, inserted a short paper on Meso- 

 type. In this paper he distinguishes three distinct species from 

 the structure of the crystals, to which he gives the names of 

 mesotype, needlestone, and thomsonite. I have little doubt 

 that Mr. Brooke's mesotype is the natrohte of Fuchs and Geh- 

 len. IVeither the specific gravity indeed, nor the measurement 

 of the angles of the crystal, as given by Mr. Brooke, coincide 

 with the previous determinations of Fuchs and Gehlen. But I 

 am disposed to ascribe this to the imperfection of the crystals in 

 the possession of these latter gentlemen, and to the superior 

 dexterity of Mr. Brooke in measuring angles by means of the 

 reflecting goniometer in consequence of his great practice. His 

 goniometer too is, I believe, more sensible than the common 

 ones, being capable of estimating a variation not exceeding 1'. 

 Perhaps the specific gravity of Fuchs, 2*256, is nearer the truth 

 than that of Mr. Brooke, namely, 224. 



The needlestone of Brooke is obviously the skolezite of Fuchs 

 and Gehlen, and the differences that occur in their descriptions 

 must be ascribed, I believe, to the same causes as those which 

 I have stated when speaking of natrolite. 



I was of opinion at first when 1 perused Mr. Brooke's paper 

 that the needlestone of Dumbarton, which Mr. Brooke has done 

 me the honour to distinguish by my name, was the mesolite of 

 Fuchs and Gehlen ; but a careful examination of it, which I 

 have just finished, has satisfied me that this first impression was 

 ill founded, and that it is entitled to rank as a distinct species. 



The only specimens which I have seen are from Kilpatrick, 

 near Dumbarton, the same place from which I\Ir. Brooke 

 obtained his specimens. The crystals are beautifully white, 

 seem perfectly pure, and are of a considerable size. It occurs in 

 considerable quantity in the amygdaloidal rocks, near Kilpatrick, 

 from which it is easily extracted by blasting the rocks with gun- 

 powder. Some years ago the mineral was plentiful. It has 

 now become scarce, though I have no doubt that if a few more 

 rocks were again blasted, it would be found as abundant as ever» 

 I have not been able to meet with a single crystal terminated by 

 a pyramid, so that I am unable to add any thing to what Mr. 

 Brooke has said respecting its crystalline form. It seems to be 

 the only mineral hitherto found to which Haiiy's mesotype cor- 

 responds nearlj'. Perhaps, therefore, it might be proper to 

 confine the term mesotype to this species, and to distinguish 

 the other two by the names natrolite and skolezite already given 

 them by Fuchs and Gehlen. Unless this be done, the name 



