346 ARGILLACEOUS SCHIST. 



occurs in two modes ; in the one, forming very 

 extensive tracts of country ; in the other, alter- 

 nating in small proportion with many other 

 rocks, and often in a very intricate manner. 



In this country, where it is found occupying 

 great spaces, it is generally the uppermost rock 

 of the primary strata, being immediately followed 

 by the secondary class. The same remark has 

 been made in other countries ; but, in many in- 

 stances, there is reason to suspect that these ob- 

 servations have been biassed by theoretical no- 

 tions respecting the place which it is conceived 

 necessarily to hold in the order of nature. In 

 other cases, it alternates with one or more of all 

 the primary strata ; often also lying in the imme- 

 diate vicinity of granite, as it does in Cornwall. 

 In Scotland however these alternations chiefly 

 occur where it occupies a small space in compa- 

 rison with the other rocks among which it lies* 



In this manner it is found accompanying 

 gneiss, micaceous schist, quartz rock, and red 

 sandstone ; and, in the latter case, it sometimes 

 appears to form a necessary part of the series, as 

 already noticed under that head. 



