194 Mr .Brooke on Mesotype, Needlestone,and Thomsoni'te. [Sept. 



The prisms are much longer in proportion than those of the 

 mesotype, and the natural planes smoother and more brilliant, 

 those of the mesotype being striated longitudinally, and aifording 

 comparatively imperfect reflections. 



The primary form of the needlestone, fig. 9, is also a right 

 rhombic prism, but measuring 91° 10' and 88° 40', M on M' 

 being the acute angle. 



It differs also in its chemical composition from mesotype, 

 which, according to Berzehus, contains no lime, while the needle- 

 stone does contain it. 



If the planes f, c', e, e', be the result of a decrement by one 

 row on the terminal edges of the prism, the height of the prism 

 will be to one of those edges as 1 to 2. 



Measurement of M on M' 88° 40' 



Mon C 116 30 



I believe it was ascertained some time since by Dr. Wollaston 

 that this substance differed from the mesotype both chemically 

 and crystallographically. 



Thomsonite, from Dumbarton . 



This substance is found in the neighbourhood of Kilpatrick, 

 near Dumbarton, and has for its primary form a right rectangular 

 prism. 



The crystals I have examined are of the form fig. 10, but 

 they are not sufficiently perfect to afford the necessary measure- 

 ments for determining the dimensions of the prism with accu- 

 racy. It is, however, nearly square, and the height equal to 

 nearly four times the lesser terminal edge if the plane c, be. 

 produced b^/ a decrement by one row on the greater edge of the 

 terminal plane. 



The measurement of M on P is 90° 00' 



M T 90 00 



M a about 135 20 



a fl' about 90 40 



The cleavages parallel to M and T are effected with great 

 facility, and the planes afford very distinct reflections. 



H. J. Brooke. 



