306 Mr. MuRCHisoN on the Coal-field of Brora in Sutherlandshire, 



unite. The lower part of this cone and bed of the stream are composed of 

 the granitic rock ; while the sides are covered by fragments of sandstone and 

 shale, derived from the beds which rise from the ravine. (See Section^ 

 PI. XXXI. fig. 2.) 



It may now be remarked that the uniformity of dip which prevails in 

 that part of the district of Brora where coal is worked, is not observable 

 where the superior strata are in immediate contact with the granitic rock. 



From Portgower to Helmsdale, the coast hills are again covered with dilu- 

 vium, concealing at that place the strata on both banks of the river ; but the 

 shelly limestone is extended into ledges on the north bank of the harbour, 

 and the worn surfaces of some of these beds expose a mass of flustrae, mingled 

 with univalves and spines of echini. 



Navidale. 



The granitic mountains retire gradually from Helmsdale, and again advance 

 to the coast at the Ord of Caithness, thus inclosing this little valley. The 

 solitary House of Navidale is placed upon a green knoll, flanked by narrow 

 glens, and overlooking a small pier. The base of the cliif is composed of the 

 shelly limestone, which very much resembles certain beds of the oolite near 

 Bath. Above the limestone, and immediately below the House of Navi- 

 dale, the sandstone is in contact with the granitic rock, the latter in various 

 stages of decomposition, and the former highly inclined. In proceeding to 

 the N. E., the place of the shelly limestone on the shore is occupied by a con- 

 glomerate much more coarse than that near Portgower, and apparently de- 

 rived from all the strata which have been described as occurring from Kintrad- 

 well to Navidale ; viz. micaceous shales, calciferous grits, and indurated lime- 

 stone and sandstone. The admixture of finely comminuted shells disappears, 

 but an occasional ostrea, or other fossil of firm texture, is still included between 

 the fragments, which are united by a sandy calcareous cement ; the whole mass 

 bearing much the appearance of an old and weathered wall. 



The indurated shales of this conglomerate might, in hand specimens, be 

 mistaken for grauwacke : but, upon a careful inspection of the whole coast, 

 and, after observing the change of character w hicli occurs in the beds at and 

 near their contact with the granitic rock, a conviction follows, that, in propor- 

 tion as the disturbing causes have become more active, a breccia has been 

 compounded from the whole suite of the superior strata of the deposits upon 

 this coast, and that the portions of it which resemble grauwacke are only the 

 sandy shales of the oolitic series in an altered state. 



Other geologists may be of opinion that some of these indurated shales with 



