LOWEST RED SANDSTONE. 399 



reaching to a dimension of many feet, and des- 

 cending even to that of an inch. They are some- 

 times separated, merely by planes, without any 

 intermediate substance, or change of structure. 

 In other cases, the separation of the beds is ef- 

 fected by intermediate laminae of shale, or clay, 

 or by alternating beds of the former substance, 

 or of limestone. Not unfrequently also, the dis- 

 tinction is produced by mere change of structure, 

 or by a difference in the magnitude of the frag- 

 ments which compose the proximate beds, or 

 the parts immediately in contact. In a few in- 

 stances, the separation of the beds is caused by a 

 lamina of parallel mica ; and, in this case, it is 

 not unusual for the sandstone to present a laminar 

 structure. 



Strata of this sandstone, like those of the pri- 

 mary rocks, are not necessarily of equal thickness 

 throughout ; but are, on the contrary, often ex- 

 tenuated till they disappear In the same man- 

 ner, they are sometimes curved in a greater or 

 less degree ; but, in this rock, as in the secondary 

 strata in general, these incurvations never pro- 



