OVERLYING AND TRAP ROCKS. 107 



nion fekpar : but I dare not propose a change, until 

 a more perfect knowledge shall allow us to reform a 

 nomenclature which will gain little by partial alter- 

 ations. How far the claystones may have been the 

 produce of a certain decomposition in the other rocks, 

 lexaminedin a former chapter, ami need but remind the 

 reader of it, here : but if what I then showed to be a 

 fact is a common or general one, there is a real trans- 

 ition throughout the whole of these simple rocks or 

 bases. The great diversity of colours in these rocks 

 is applicable only to the distinction of varieties. In 

 common clay, equally diversified, these- are neglected; 

 while the series exactly resembles that of the clay- 

 stones and compact felspars. The prevailing tints 

 are various tones of grey, red, and yellow, passing 

 towards white ; and the same substance occasionally 

 partakes of every colour. In the Syenites and green- 

 stones, where these minerals form a part, it has been 

 the general and careless practice, to class the dark 

 and grey varieties with the latter, and the pale, red- 

 dish, or yellowish ones, with the former ; as the dark 

 hard claystones have been confounded with basalt. 

 Thus, under the same relative proportions of horn- 

 blende and compact felspar, the one has been called 

 Syenite and the other greenstone, because the colour 

 of the latter mineral was in the one case, pale yellow, 

 and in the other, iron grey. The Classification has 

 attempted to rectify this confusion ; produced by as- 

 suming a distinction which was superficial, and 

 neglecting the essential one. The iron appears to 

 exist in three states, that of a carbonat, a protoxyde, 

 and a peroxyde. In the first case, the rock is pale ; 

 the presence of the iron being discovered only by the 

 rusty colour of the decomposed surface ; by which 

 criterion alone it is distinguished from compounds 



