88 PKOP. J. W. JUDD ON TEETIAEY 



The order of separation of the several minerals in these rocks is 

 shown not to have resulted from purely chemical causes, but to have 

 been controlled by the conditions under which consolidation has 

 taken place. In the most deeply-seated varieties, the iron-oxides 

 enter into complete combination with the silicates, and in other 

 cases there is a progressive increase in the quantity of magnetite 

 which is separated, according to the proximity to the surface at which 

 consolidation has taken place. 



Although some of the rocks exhibit a remarkably fresh and 

 unaltered condition in their component minerals, others display the 

 effects of both deep-seated and surface-action in the most instructive 

 manner. There is no essential difference between these altered 

 forms and the similar rocks of pre-Tertiary age. The nature of the 

 changes and the causes by which they have been brought about are 

 shown to vary greatly in different cases, but especial attention is 

 drawn to the deep-seated action to which the name of schilleri- 

 zation has been given, and to the effects produced by the percolation 

 of atmospheric waters. 



EXPLANATION OF PLATES lY.-YII. 



In the first three Plates an attempt has been made to show the 

 very gradual transition of the most highly crystalline varieties 

 of these rocks into the perfectly vitreous types. Plate IV. is devoted 

 to the illustration of some of the most characteristic and typical 

 varieties of the gabbros, and of their approximation towards the 

 dolerites. Plates Y. and YI. illustrate the passage of the dolerites 

 into basalts and magma-basalts. In each of these Plates the 

 figures on the left hand indicate the structure of rocks which 

 have cooled down with the minimum of internal movement and 

 strain (ophitic structures and glass with skeleton -crystals) ; while 

 the figures on the right illustrate the types in which there has been 

 much internal movement and strain during consolidation (granulitic 

 structure and glass with granular microliths and magnetite dust). 

 It has not been found practicable to give representations of the most 

 largely crystalline types of these rocks, for in such the individual 

 crystals are often more than an inch in diameter ; but the series 

 begins with the gabbros of medium-sized grains. These are repre- 

 sented with a very low magnifying power (10 diameters only), and 

 as the crystalline character diminishes, the magnifying power applied 

 to the sections has had to be increased. In drawing the objects it 

 has been found necessary to diminish them to about one third of 

 the size they appear at the position of distinct vision (10 inches), 

 and the enlargement is therefore indicated by the figures y*, 2|-, y , 

 &c. The actual size of the objects represented will of course be 

 found by dividing the linear measurements in the figures by the 

 quotients of these several fractions. The least magnified sections 

 are represented within rectangular outlines, the degree of departure 

 from this indicating the progressive increase of the amplification, the 

 most highly magnified sections being represented in circles. 



