106 



Practically, there are few cases in which ge- 

 neral conclusions with regard to these circum- 

 stances can be safely drawn from the measurement 

 of one part of a stratum. 



The positions of strata with regard to each 

 other, or to the neighbouring rocks, are various. 



When in contact with granite, or other irre- 

 gular rocks, they are rarely, if ever, parallel to its 

 surface, for any considerable space, or adapted to 

 the external form of the mass. 



More commonly, such an irregular rock is 

 found in contact with many successive associated 

 strata, or meets them at some angle to the planes 

 of their stratification. In such cases they are fre- 

 quently contorted, or fractured, and penetrated by 

 veins which are connected with the irregular 

 rock and formed of the same materials. In some 

 rare instances they are also broken into fragments 

 of different dimensions, which are found insulated 

 in the irregular rock. 



When strata follow each other, they are very 

 often parallel for a considerable space ; or as nearly 

 parallel as the inequality of their thickness permits. 



