in the Old Red Sandstone. ' ' ■{ 8$ 



mass, or those nearest the sandstone, frequently present a brecciated 

 structure. Throughout the rock, but chiefly towards the lower 

 parts, thin interrupted layers of sedimentary matter, in the form of 

 slate clay, and slaty sandstone, often of considerable hardness, 

 may frequently be observed, conformable in dip with the more 

 perfectly developed sedimentary rocks above and below the 

 amygdaloidal mass. It seems highly probable from such appear- 

 ances, that while volcanic matter was furnishing materials for a 

 subaqueous amygdaloid, in successive portions, the water was 

 likewise, from lime to time, depositing its mud, resembling, in 

 many respects, that with which it had previously been forming 

 the inferior sandstone, to be afterwards considered. 



The amygdaloid contains irregular beds of porphyry, claystone, 

 compact felspar, greenstone, and clinkstone. The latter rock 

 furnishes excellent compact durable materials for building, and 

 frequently assumes the prismatic form. It readily splits with the 

 hammer, in the direction of the dip, and in this manner exhibits its 

 laminated structure. The latter character, however, is displayed with 

 greater distinctness in those portions of the rock which have been 

 long acted on by the weather, and in which the parallel layers are 

 so numerous and well marked, as to leave no room to doubt its 

 sedimentary origin. Some, perhaps, would, even in the face of 

 such evidence, pronounce it a lava, while I am convinced that it 

 is altered slate clay. This bed of amygdaloid, with its subordinate 

 materials, constitutes the range of the Ochils, rising towards their 

 western extremity, at Bencleugh, to the height of 2450 feet above 

 the sea. 



3. Gray Sandstone, 



This rock seems originally to have been derived from the 

 decomposition of clay-slate, and mica-slate. In its upper part, 

 it appears in the form of slate clay, in which the layers are of 

 considerable thickness, and usually receives the denomination of 

 calmstone. In the lower portions of the series, where the sandstone 

 occurs, the beds are of considerable thickness, and furnish excellent 

 materials for building. In some cases the sandstone is thin, slaty, 

 and has been emploj'^ed for roofing. The whole abounds in mica. 

 This deposit seems to be identical with the Arbroath pavement, 

 and with the inferior sandstone of Mr Lyell's Section of Forfar- 

 shire, published in the Transactions of the Geological Society of 

 London, Vol. II. second series, PI. 10. There is equally little 

 reason to doubt its identity with the schists of Caithness, de- 

 scribed by Professor Sedgewick, Geological Transactions, Vol. III. 

 second series, PI. 14. 



4. Old Red Sandstone. 



This rock exhibits the usual beds of sandstone and conglomerate. 



