218 LOWEST, OR OLD RED, SANDSTONE. 



the mountain limestone, it includes other substances 

 inferior in quantity, and irregular in their recurrence. 

 The chief of these are argillaceous schists, though 

 considered as shales under the adopted classification. 

 These are sometimes undistinguishable from the pri- 

 mary graywacke schists, when separated from their 

 connexions, while, at others, they are mixed with the 

 ordinary materials of the sandstone^ so as to present 

 transitions into it. In other cases again, they are as 

 fine and compact as the primary slates ; occasionally 

 resembling even the green chloritic varieties. Lastly, 

 beds resembling the ordinary shales of the upper 

 strata, occur also in this deposit ; being simply argil- 

 laceous, or argil lo-calcareo as, or so charged with iron 

 as to be red or yellow, and thus finally passing into 

 argillaceous ironstone. Beds of limestone are some- 

 times also contained in the red sandstone, though sel- 

 dom extensive, and rather forming irregular masses or 

 limited strata than continuous alternations. At times, 

 they are mixed with the fragments of the surrounding 

 conglomerate, at others divided by laminae of shale, 

 and contaminated with clay, siliceous matter, and 

 oxydes of iron ; occasionally also containing organic 

 remains. Coal also occurs in it, as elsewhere noticed, 

 though rarely, and seldom in any quantity ; yet so as 

 to be occasionally wrought in a limited manner in 

 Scotland. But in reading such reports, let the geo- 

 logist not forget how often the red marl has been con- 

 founded with it. 



I must now point it out as a general fact in my ex- 

 perience, and, as I hope to show, an important one, 

 that wherever this sandstone is highly indurated, its 

 shales, whether fine or coarse, are identical with the 

 primary schists, and that the argillaceous beds among 

 the softer ones resemble the shales and clavs of the 



