66 The Founders of Geology LECT. 



other in age and other characters. The first of these 

 classes includes the younger lava - streams, not yet cut 

 through by running water, and still connected with their 

 parent cones. The second embraces those lavas which bear 

 decomposed earthy materials on their surface, and from 

 which their original craters have disappeared. In the 

 third class are ranged those lavas which have been reduced 

 to detached outliers separated by valleys; while in the 

 fourth, some isolated masses are placed which Desmarest 

 thought had been "melted in place," or erupted where 

 they now appear. 



The third part of the memoir, though read with the 

 second part in 1771, was not published until 1777. In 

 this essay the author discussed the basalt of the ancients, 

 and the natural history of the various kinds of stones to 

 which at different times the term basalt had been applied. 



It is interesting to follow the slow elaboration of his 

 views through his successive memoirs. We must remember 

 that, during these busy years, his time and thoughts were 

 chiefly taken up with the inquiries into industrial develop- 

 ment which the Government of the day had entrusted 

 to him, and which necessitated frequent and prolonged 

 journeys, not only in France, but in other countries of 

 Europe. He felt that the great questions in physical 

 geography which specially occupied his attention could 

 best be studied in Auvergne. He returned to that region 

 at every available opportunity, revisiting again and again 

 localities already familiar to him, and testing his deduc- 

 tions by fresh appeals to nature. Four years after his 

 great monograph on the origin of basalt had been read to 

 the Academy of Sciences, he presented another essay, 



