402 PROF. J. W. JITDD 0?f THE TEETIAKT AND 



I. II. III. 



Silica 41-74 42-17 42-04 



Total iron as ferric oxide 1 1 "58 10-36 10-92 



Alumina 3-08 3-97 3-48 



Magnesia 26-83 30-65 



Lime 475 3-77 



Manganous oxide *70 



Soda and potash 4-85 1-90 



Water and carbonic acid 7'78 7-19 7-67 



Ferrous oxide 3-63 2-01 2-40 



Erom a discussion of the results which, the analyst considered 

 most reliable, he gives the following as the composition of the rock : — 



Silica , 42-10 



Alumina 3-28 



Ferric oxide 8-27 



Ferrous oxide 2'13 



Manganous oxide '70 



Lime 3-77 



Magnesia 30*65 



Soda and potash 1-90 



Water and carbonic acid .... 7-73 



100-53 



The specific gravity of the rock was determined for me by Mr. 

 Grenville Cole on the least altered specimen I could procure, and 

 proved to be 2-82. 



The proportions of silica, alumina, magnesia, and water indicate 

 that the rock belongs to the class of the ultrabasic rocks, and that 

 it is in an altered and hydrated condition. 



The macroscopic appearance of this rock is very peculiar. It most 

 nearly resembles some of the so-called '-' Schiller-spars," especially 

 the paler varieties known as diaclasite ; but closer examination 

 shows that, as Prof. Heddle pointed out, it is a perfectly unique rock. 



The most conspicuous mineral in the rock has a micaceous 

 appearance, with a pale bronzy-yellow colour and a submetallic 

 lustre. The rock breaks in large flat planes^ along the cleavage of 

 this mineral, and in some cases these planes of fracture cut one 

 another at such sharp angles as to give the impression that the rock 

 is made up of a number of large prismatic crystals. At the request of 

 Mr. L. Fletcher, to whose advice and assistance I am greatly 

 indebted in connexion with these studies, Mr. Miers, of the j^atural 

 History Museum, made a series of measurements with the contact 

 goniometer, the results of which were so discordant as to prove 

 that these planes are neither crystal-faces nor planes of cleavage. 

 The conclusion was confirmed by the study of large sections of the 

 rock, which show that the micaceous mineral lies in different positions 

 within each of the pseudo-crystals. 



Prof. Heddle regarded the micaceous mineral as talc *, and he 

 states that the rock is made up of that mineral, with augite, some 



* 3Iineralogical Magazine, vol. t. (1884) p. 260. 



