OCCASIONAL ROCKS. 539 



ranked as pitchstones, so as to have produced 

 great confusion, and to have introduced most 

 important errors into the geological history of this 

 substance. These particulars will be discussed 

 at more length in the preliminary remarks to the 

 catalogue of that rock ; and there will then be 

 proved, what need only be mentioned here, that 

 it occurs indiscriminately both in the primary and 

 the secondary classes. 



Nearly the same observations may be made 

 on siliceous schist, which also occurs indiscrimi- 

 nately, although generally with some difference 

 of character, in both classes. Geologists, who 

 have admitted this rock into their syst ms, have, 

 in some measure, felt the necessity of making 

 two divisions of it ; and have accordingly divided 

 it between their primitive and transition classes. 

 But they seem either to have overlooked its fre- 

 quent situation in the secondary class, or else to 

 have confounded it, when it occurs there in an 

 unquestionable manner, with some varieties of 

 those trap rocks with which it is invariably asso- 

 ciated. In this way, much confusion has been 

 introduced into geological descriptions: since 



