310 THE CANADIAN NATURALIST. [Sept. 



Befure procecdinii; to explain the foregoing table, it ma}- be 

 mentioned that no new names have been used in its construction ; 

 that names to which definite ideas as to mineralogical constitution 

 are attached, have been, as much as possible, excluded. Such 

 names as trap, greenstone, and melaphyre, which have been, in the 

 early history of the science, much abused and misapplied, and 

 more recently condemned as useless for the purpose of indicating 

 any special rock, are introduced into our table', and advantage- 

 ously used in designating the families of rocks to which they 

 were originally applied. If it were made a rule in the science to 

 exclude from it all names which have been at one time or other 

 misused, very few petrological terms would escape obliteration ; 

 and the fact that the names above mentioned, in spite of their 

 condemnation by some lithologists, continue in common use, suffi- 

 ciently proves that they possess a certain degree of usefulness and 

 applicability. 



It will be observed that in the table the terms basic and basous, 

 silicic and siliceous, are used in a manner analoirous to that in 

 which the stron2;er and weaker bases and the strono-er and weaker 

 acids are indicated in chemical nomenclature. A baste slate 

 always contains a larger percentage of bases than a baso^.s' one, 

 and a silictc porphyry in the same way contains more silica than a 

 h'lViceous one. [t will next be observed that we have in the table 

 eight different horizontal series of rocks, or rather rock families, 

 corresponding to the eight different varieties of texture which have 

 been before particularized. On passing in each of these series 

 from left to right, we pass from the basic to the siliceous extremes, 

 through rock families gradually increasing in silica contents, as 

 the figures at the head of the vertical columns shew. With this 

 increase in the amount of silica a corresponding change in the 

 nature of the bases with wliicli it is combined takes place. 

 Towards the basic extreme these are principally magnesia, lime, 

 and protoxide of iron ; but as the silica increases these bases 

 diminish, and alumina with the alkalies increase until, at the 

 silicic extreme, alumina and potash become the preponderating 

 bases. We have also in the table five different vertical series, 

 among which the neutral, basic and silicic groups already referred 

 to, occupy places in the middle and at the sides, while the inter- 

 mediate groups, which were also mentioned above, and which 

 have been called the basous and siliceous rocks, occupy positions 

 iramediatelv to the left and ri2:ht of the central column. The 



