346 



PROFESSOR HEDDLE ON 



The crystals of andalusite are from half to nearly one inch in length, by about one- 

 third of that thickness, and it is to be remarked that though for the most part here 

 disposed in layers, they are very far from invariably disposed upon their sides, as 

 regards the rock bedding, though that position dominates. They are ash-grey in 

 colour, and in section and even to the eye a central lozenge-shaped tessela of darker 

 and clearer shade of colour is seen ; while the whole substance of the crystals is also 

 seen to spangle with crystals of biotite. These are equal in size to those generally 

 occurring in the rock, are disposed like these in all directions, and are not very 

 markedly fewer in number. 



The analysis of these was made on crystals freed absolutely from the inclosing rock, 

 and with even some portion of their outer surfaces removed to ensure as great purity 

 as possible. They yielded — 



On 1*3 grammes- 



Silica, 

 Alumina, 

 Ferric Oxide, . 

 Ferrous Oxide, 

 Manganous Oxide, 

 Lime, 

 Magnesia, 

 Alkalies, . 

 Water, . 



52-538 



39-314 



1-094 



3-267 



•461 



•861 



•846 



trace 



1-11 



99491 



Loss, -238 per cent, of water in the bath. 



This result shows a very considerable intermixture with all of the ingredients of the 

 rock, notwithstanding which the crystals are hardly affected by the knife, and have a 

 vitreous lustre. 



Three theories have been advanced to account for the presence of the crystalline 

 constituents of clay-slates, for they occasionally bulk so largely as to entitle them to the 

 name. According to the first of these theories, the crystals in question are regarded 

 as the product of chemical action in the ocean in which the original material was 

 deposited. The second theory attributes and confines the formation of the crystal- 

 line minerals to processes of metamorphism which have taken place subsequent to the 

 solidification of the rocks. The third theory refers them to an aggregative action going 

 on in the still plastic clay-slate mud prior to its solidification. 



The first of these theories has been maintained by Credner ; but against it 

 numerous arguments have been adduced, and especially the difficulty of supposing an 

 ocean capable of depositing from its waters at successive periods minerals of sue! 1 

 different chemical composition as actinolitc, andalusite, chlorite, etc. 



