PETROGRAPHY. 327 



he recognised numerous minute foreign bodies and inclusions. 

 But these authors failed to make sufficient impression upon 

 contemporary thought. Petrography continued to be con- 

 ducted for the most part along the old lines ; in Germany the 

 best known teachers of petrography were Rose, Cotta, Nau- 

 mann, and Rath; in France, Delesse, Durocher, and Fournet. 

 Naumanri's Lehrbuch contains an admirable representation of 

 the state of petrography in 1850. But, instead of the sub- 

 divisions then customary, Naumann differentiated rocks chiefly 

 according to their origin as crystalline, clastic, hyaline, poriform, 

 zoogene, and phytogene. 



In the following decade, the interest of petrographers was 

 chiefly directed to the chemical side. Until that time, geology 

 had troubled little about chemistry. The foundations of 

 geology had been laid without the assistance of chemistry; 

 among the leading geologists of the heroic period, only 

 Hutton and De Saussure were learned in chemistry, and they- 

 had not seemed to find much use for their intimate knowledge 

 of that branch of science. Cordier had in 1815 applied 

 hydrochloric acid for the determination of certain constituents 

 of rocks, and Gmelin in 1828 had made an analysis of 

 phonolite, separating the elements that were soluble in hydro- 

 chloric acid from those that were insoluble. But a purpose- 

 ful chemical investigation of rocks was first attempted by 

 Bischof and Bunsen. 



Gustav Bischof (ante, p. 217), the founder of Chemical 

 Geology, was much more a chemist than a geologist, and 

 although his lack of sound geological knowledge could not 

 affect his experimental chemical researches on rocks, it proved 

 detrimental when he came to draw generalisations from his 

 results. In the first volume of his Text-book of Chemical and 

 Physical Geology, Bischof begins with the consideration of the 

 water on the surface of the earth and in internal- cavities and 

 joints ; after a detailed description of springs, he turns his 

 attention to their temperature, their chemical ingredients, etc., 

 and to the chemical changes which are set up in the rocks 

 when water is brought into contact with them. The second 

 volume is a complete chemical mineralogy and petrology, in 

 which the mode of origin of the rocks receives a large share 

 of attention. When he reviews his facts, Bischof arrives at 

 conclusions of an ultra-Neptunistic tendency and quite 

 erroneous. The work is of high value on account of the 



