1870.] MACFARLANE — ON CRYSTALLINE ROCKS. 165 



species of texture given above, those have been placed nearest to 

 each other which are most prone to pass into each otlicr by 

 modifications of texture. 



To account satisfactorily for these variations of texture among 

 original rocks is no easy matter ; but if the facts already given, 

 as regards the solidification of artificial silicates, have any value 

 as applied to lithogy, they would lead us to suppose that the 

 coarsely schistose rocks solidified very slowly during the lapse of 

 great intervals of time and under the influence of widely extended 

 movements of the crystalline, but still fluid mass; that the 

 coarsely granular rocks solidified very slowly, but in comparative 

 rest; that porphyritic and small-grained rocks cooled more quickly 

 than coarse granites, although crystallisation evidently took place 

 while they were in a plastic condition ; that fine-grained schistose 

 rocks solidified while in motion, but are the products of compara- 

 tively rapid cooling ; that porous trachytes cooled rapidly, but in 

 comparative rest; that very cavernous rocks came into contact 

 with water during cooling, and we may suppose that, where that 

 element was present in great quantity, many original rocks 

 underwent disintegration while their solidification was in process, 

 giving rise to the tufaceous series of derived rocks. Many of 

 those generalisations are supported by observations recently made 

 on the microscopic structure of rocks to which, however, it is 

 impossible here to refer. 



(To he continued.^ 



AQUARIA STUDIES. 

 (Part II.) 



By a. S. Eitchie. 



In the last number of this journal a description was attempted 

 of some of the difi'erent representatives of animal life contained in 

 our aquarium, of what may be termed its visible beauties, that 

 is, such creatures as may be seen with unassisted vision. The 

 present sketch is connected still further with its denizens, as 

 beautiful in their structure, and, notwithstanding their minute- 

 ness, no less wonderful in their design. 



The unassisted eye can only look at relatively few of the 

 creator's works: it cannot enter the inner shrine of nature's 



