ON THE PLANT-BEARING CHERTS AT RHYNIE, ABERDEENSHIRE. 207 



The area was sui'veyed in detail by the Geological Survey, and the 

 results, which confirm the conclusions previously arrived at by Sir 

 Archibald Geikie as to the order of succession of the strata, are repre- 

 sentfed in the one-inch map (Sheet 76) published in 1886, and are briefly 

 described in the explanatory memoir to that sheet published in 1890. 

 The classification adopted in the memoir is given below - : — 



(5) Dryden flags and shales. 



(4) Quarryhill sandstones. 



(3) Tillybrachty sandstones with volcanic zone. 



(2) Lower red shales with calcareous bands. 



(1) Basal breccia and conglomerate. 



Early in 1910 Dr. Mackie became aware that a narrow strip of 

 sedimentary and volcanic rocks occurs to the west of the boundary 

 fault as laid down on the Geological Survey Map (Sheet 76). This 

 strip is situated about a quarter to half a mile due west of the village 

 of the Muir of Ehynie, and extends both north and south of the Rhynie 

 and Cabrach road. These beds present a much more ancient-looking 

 facies than the Old Red Sandstone strata east of the fault, and were 

 found on examination in detail to include cherts, silicified grits and 

 conglomerates, together with a very acid andesite or rhyolite, which 

 also shows silicification in places. The results were described by 

 Dr. Mackie in a preliminary paper communicated to the British 

 Association at the Dundee meeting in 1912,^ and in greater detail 

 in a paper read before the Edinburgh Geological Society in 

 November 1913.* In addition to the series there described, Dr. 

 Mackie brought to light a distinct band of volcanic ash just 

 above the rhyolite and between it and the ' Upper Grit ' of the same 

 series. Late in 1912 — too late for inclusion in the British Association 

 paper — numerous blocks of a fine black chert were discovei'ed by 

 Dr. Mackie lying loose on the surface or built into the stone dykes 

 along the sides of the adjacent fields. These appeared to radiate from 

 a centre about seventy-five yards east of the bend of the road leading 

 to Windyfield farmhouse. They were traced eastward for about three 

 hundred yards, but up to that time they had nowhere been found in 

 place. Their cherty character was at first the main point of interest, 

 and for that reason, in the absence of field evidence of their strati 

 graphical position, they were naturally supposed to belong to the 

 silicified ' Older Series ' to the west of the Old Red Sandstone boundary 

 fault. Between the date indicated and October 1913 numerous micro- 

 sections of the chert were examined by Dr. Mackie, which proved 

 to be exceptionally rich in plant remains in a remarkably perfect 

 state of preservation. These were at once placed in the hands of 

 Dr. Kidston for description in detail, and a brief account of them 

 drawn up by him is appended to this report. Dr. Mackie believes that 

 the microscopic sections of the plant-bearing cherts also show remains 

 of small Crustacea, which are still under investigation. 



" Explanation of Sheet 76 {Mem. Geol. Siir.), p. 27. 

 3 British Assoc. Report, Dundee 1912, p. 467. 



* ' The Rock Series of Craigbeg and Ord Hill, Rhynie, Aberdeenshire,' 

 Trans. Edin. Oeol. See. vol. x. part 2, p. 205. 



