Prof. Hull — Structure of Porphyry. 



451 



in. F'ig. 1. Here they assume a stellate, or plumose arrangement, 

 branching out from certain centres placed at intervals around the 

 sides of the original cell, now filled in with chlorite and calcite. 

 This structure I regard as due to polarity, the magnetite grains 

 having arranged themselves side by side somewhat in the manner in 

 which steel dust will arrange itself between the opposite poles of a 

 magnet. Dr. Reynolds, however, suggests that the structure is 

 similar to the dendritic form assumed by manganese ore between the 

 laminse of sandstone or shale, and which he has shown to be due to 

 magnetic force. In this case he supposes the cleavage planes of the 

 calcite to have afforded the necessary conditions.. 



Fig. 1 .— Cell in the porphyry 'lined with chlorite and filled with calcite. Several 

 stellate prominences formed of minute grains of magnetite project from the 

 walls of the cell into the interior, and are imbedded in calcite. Mag. 25 diams. 



Chlorite; — This mineral is abundantly disseminated throughout 

 the base. It also occurs in small irregular grains, or (along with 

 calcite) fills in cracks and cavities in the rock. It is of a pale leek- 

 green colour, structureless, but often inclosing minute cells, probably 

 gas or air bubbles, as shown in Fig. 3. It is to be presumed that 

 this mineral is in every case of secondary origin, and is not directly 

 the product of igneous fusion. It has therefore been introduced by 

 the agency of water, which has permeated the whole mass of the • 

 rock through channels inconceivably narrow, and which can only be 

 revealed by the aid of a high microscopic power. In the section of 

 the cavity (Fig. 1) which has been filled in by this material and 

 calcite, the chlorite forms a partial lining, interposed between the 

 walls of the cavity and the calcite which fills the interior, so that it 



