INTRODUCTION. 83 



of these rocks, and his clear exposition of their volcanic 

 nature. 



After the publication in 1 788 of Werner's work on the 

 occurrence of basalt at the Scheibenberg Hill, the difference 

 of opinion between these two geologists began to assume a 

 more personal aspect, and unfortunately ended in a rupture 

 of their friendship. 



Voigt published several important papers on the geology of 

 Thuringia in later years, chiefly in mineralogical journals, and 

 he was also the author of the first practical Text-book of 

 Geognosy (Weimar, 1792). In the description of the rocks 

 and the order of rock-formations in the crust, Voigt follows 

 Werner's teaching, but he has a more just appreciation of the 

 causes of volcanic phenomena and the origin of volcanic 

 rocks. 



His last large work was entitled Attempt at a History of 

 Coal, Brown Coal and Turf (Weimar, 1802-5). This con- 

 tains, in addition to the geological data, practical advice on the 

 determination of workable coal-seams, and the industrial uses 

 of the various kinds of combustible deposits. 



A detailed account of several localities in the Thur- 

 ingian Forest was also given by Johann Ludwig Heim, a 

 Privy Councillor in the Duchy of Meiningen. Heim (1741- 

 1819) was tutor to the Princes of Meiningen, and during 

 occasional journeys he made a large mineralogical collection, 

 and wrote a number of papers compiled into one larger work, 

 Geological Descriptions of the Thuringian Forest (Meiningen, 

 1796-1812). These are distinguished by the independence 

 of his views, acute powers of observation, and his clear 

 descriptions; but there is no geological map, and the 

 stratigraphical details are only illustrated by rough sketches. 

 Hence the work, careful though it was, never received much 

 recognition, and was much less instructive in character than 

 that of Voigt. 



Heim referred the origin of the primitive rocks to chemical 

 crystallisation from an indefinite mixture or "fluidum," 

 possibly gaseous in constitution. He allowed that the slates 

 and greywackes ("transitional rocks " of Werner) might have 

 been precipitated from a watery fluid, but he thought it 

 impossible to trace any difference in the ages of the various 

 precipitates. His idea was that all these rocks are arranged 

 in the crust as spherical or elliptical masses whose kernel is 



