156 COMPACT FELSPAR. 



and consisting, chiefly, of that mineral, sufficiently 

 known by mineralogicai descriptions. If most fre- 

 quently simple, it sometimes also contains interspersed 

 particles of quartz and hornblende; and, by the increase 

 of these, it passes into some of the varieties of gneiss. 



In stmie places it seems scarcely stratified ; forming, 

 rather, irregular nodules like those of limestone and ser- 

 pentine; and no farther therefore a source of trouble or 

 doubt. As lona offers a convenient example of this 

 form, I may refer to that island for such further informa- 

 tion as cannot easily be given in words, as well as for 

 the evidence of the fact itself. In the Long Island and 

 Rossshire, it is stratified, at least as distinctly as the 

 gneiss which it accompanies ; and it may thus also be 

 seen and studied in the neighbourhood of Pol Ewe 

 and Loch Greinord, and on the shores of Loch Maree. 



With some very trifling exceptions, in which it is 

 particularly connected with granite, it seems exclur 

 sively to be associated with gneiss, whether as an im- 

 bedded mass, or an alternating stratum. Such gneiss 

 is remarkable for containing compact felspar as an in- 

 gredient ; and the two rocks pass into each other by 

 a mutual gradation ; the one acquiring, or the other 

 losing, those ingredients on which the distinction de- 

 pends. It must still be added to its geological cha- 

 racters, that it is found also in the form of veins in the 

 same situations, frequently ramifying into filaments of 

 extreme tenuity. On decomposition, this rock forms 

 a very fine porcelain clay ; and may thus become an 

 object of research when better known. 



If it shall appear to Geologists that a new rock has 

 been unwarrantably added to the System, some other 

 mode must be discovered of classing a substance which 

 cannot be considered a mere mineral, and which 

 refuses to associate with anv other rock. 



