XXIII. — Supplementary Remarks on the Strata of the Oolitic Series, 

 and the Rocks associated with them, in the Counties of Sutherland 

 and Ross, and in the Hebrides. 



By RODERICK IMPEY MURCHISON, Esq., Sec.G.S. F.R.S. F.L.S. 



[Read Nov. 16th, 1827.] 



Having revisited the district of Brora and those other tracts in the 

 Hebrides which I described in a memoir pubUshed in the Transactions of 

 the Geological Society *, and, upon this latter occasion, having enjoyed the 

 advantage of Professor Sedgwick's company, I am enabled to make some 

 further observations, and to present many illustrative fossils, in addition to 

 those noticed in the former list ; which serve to confirm the existence and order 

 of superposition of the strata of the oolitic series. 



I must, however, preface these observations by a few remarks on the struc- 

 ture of the crystalline rocks which flank these secondary deposits on the east 



coast of Sutherland. 



Granite. 



The promontory called the Ord of Caithness, which constitutes the north- 

 eastern termination of the deposits of the oolitic series in Sutherland, has been 

 described as a "granitic rock, composed of much felspar and quartz, with a 

 green substance in a decomposed state which may have been micaf ." Now, 

 although such is the prevailing character near its junction with the secondary 

 strata, an examination carried on more in the centre of the mass has detected 

 so many examples of well crystallized mica, that this rock must be considered 

 a true granite];. 



In the previous memoir I stated, that wherever this rock comes in contact 

 with the beds of the oolitic series, the latter are compounded into a remark- 

 able breccia : and recent observation has not only fully confirmed the conclu- 

 sion which I drew from these phaenomena; viz. "that the granite of this 



* Ante, page 293. t Page 294. 



J This rock of the Ord has been described by Professor Jameson as " compact granite com- 

 posed of felspar and quartz." — Mineralogy : Scottish Isles, p. 259. 

 VOL. II. — SECOND SERIES. 3 A 



