82 THE CANADIAN NATURALIST. [Vol. ix. 



amount of calcium that of bisilicate) was immediately followed 

 by the assumption that these two silicates combined indefinitely , 

 and, therefore that all the triclinic feldspars were essentially one 

 species, and for this reputed species the name plagiodase has 

 been used. Some ground for the assumption was found in the 

 analyses of the feldspars; but how much was uncertain, because, 

 in several cases, mechanical mixtures of one species with another 

 had been ascertained to exist in crystals. Now that Des Cloizeaux 

 has proved, by optical investigations, that several of the species 

 of triclinic feldspars are really species, that is, that the combina- 

 tions of the two silicates, the tersilicate and bisilcate, are based 

 on definite ratios, as in combinations in other departments of 

 chemistry, and that there are not indefinite blendings, the term 

 " plagiodase " has become merely a synonym for " triclinic feld- 

 spar." 



The consequences to lithology of this introduction of the term 

 *' plagiodase " were unfortunately great. It was made a suffi- 

 cient definition of a rock to say that it consisted of ^- plagiodase 

 and hornblende," ^^ plagiodase and augite," and so on ; and 

 this is now common in recent memoirs on rocks. It was a con- 

 venient idea ; for an examination with the microscope is made 

 in a hundredth part of the time required for a chemical analysis. 



Now this word " plagiodase " covers compounds varying in 

 the silica afibrded by analyses from 43 to 69 per cent. ; and in 

 the alkali from all lime (20 per cent.) to all soda (12 per cent.) 

 Anorthite, the lime feldspar, is not oligoclase, even if to the two 

 a common name be applied ; they still differ 20 per cent, in 

 silica (which is one-fifth the mass), and also in the alkali present. 

 Expressions like "consisting of plagiodase and hornblende," as 

 in the definition of dioryte, have consequently an immensely wide 

 signification ; for the word dioryte is made to cover oligioclase- 

 dioryte, labradorite-dioryte, and anorthite-dioryte. 



This confounding of things thus unlike may be called simplify- 

 ing the science of lithology ; but it is a confounding of important 

 distinctions in the view of those who are interested in a definite 

 knowledge of rocks, and in the important geological questions 

 connected with their constitution. Some lithologists recognize 

 the bearings of such questions, and use the qualified terms for 

 the kinds of dioryte above cited. But the most recent turn is 

 in the other direction. Rosenbusch's learned work, the latest, 

 Bays that the rocks of the " family " of diabase consist of " plagio- 



