1870.] MACFARL\NK — O.N CRYSTALLINE ROCKS. 311 



rock families of cacli of tliCoC vertical series, although they may 

 differ widely a-; rcLrarJs their texture, all possess a similar 

 chemical composition. 'J'lie chemical nature, texture, and affini- 

 ties of any original rock or rock family are seen from tliis table at a 

 glance. Thus, porphyrite appears as the porphyritic develope- 

 ment of the siliceous group of rocks ; as less siliceous than felsitic 

 porphyry, and more so than melaphyro. Basalt is seen to be the 

 most basic member of the fine-grained order, and to contain less 

 than forty-nine per cent, of silica. The affinities of any rock may 

 be ascertained by observing the names of the rocks placed next 

 to it, for in almost every case it is into these that it is most prone 

 to graduate. 



There arc other of the general relations among original rocks 

 visible from this table than those which refer to their composition 

 texture and affinities. Not only do the rock families mentioned 

 in each vertical column resemble each other in chemical composi- 

 tion, but they also exhibit similar coincidences as regards their 

 general colour, hardness and fusibility, and gradual transitions 

 in each of these respects are found to exist from rock to rock 

 along each horizontal series. The basic rocks are generally 

 darker coloured, less hard, and more readily fusible than the 

 rocks which correspond to them in texture but differ from them 

 in containing a larger percentage of silica. On the other hand 

 the more siliceous a rock is, the lighter it will generally be found 

 to be in colour, the harder and more difficult to penetrate or 

 excavate, and the more refractory on exposure to high tempera- 

 tures. 



There is yet another physical property belonging to those 

 original rocks, in which they show a similar correspondence with 

 their chemical composition. Still speaking generally, the more 

 siliceous a rock the lighter it is, not only in colour, but in weight ; 

 the more basic the rock, the heavier it becomes. Thus it is the 

 case that, in each order of texture on passing from the siliceous 

 to the basic rocks, a gradual increase of density takes place, and, 

 on the other hand, the transition from the basic rocks to the 

 more siliceous exhibits a gradual diminution of specific gravity. 

 So constant is this relation that it may be taken advantage of in 

 determining the general composition of a rock. To take as an 



