ON THE PLANT-BEARING CHERTS AT RHYNIE, ABERDEENSHIRE. 215 



report that they have not detected any signs of contact alteration in 

 the grit when examined under the microscope. Dr. Mackie has noted 

 the occurrence of fine quartz veins between the two rocks in all his 

 microscopic sections. The rhyolite weathers into a white plastic clay, 

 with knots of less decomposed material. From its microscopic 

 characters it may be classed as an andesite. Its outcrop along the 

 road section measures 33 yards. It passes upwards into a band of 

 volcanic ash with gritty partings, followed by the ' Upper Grits ' of 

 Dr. Mackie's succession, consisting of hard, flaggy, much broken, 

 micaceous sandstones with interbedded tuffs. 



(3) At the eastern margin of the ' Upper Grits ' the beds are much 

 disturbed, and there are clear indications of faulting. Dr. Mackie 

 infers that these indications mark the position of the fault that bounds 

 the Ehynie outlier of Old Red Sandstone on its western side.* The 

 locality is 250 yards from the datum line and about 120 yards further 

 to the west thon the position of the fault laid down in the Geological 

 Survey One-inch Map (Sheet 76). 



(4) Eastwards beyond the boundary fault a continuous rock section 

 was laid bare for about 20 yards. The strata exposed (fig. 3) dip to the 

 east at angles varying from 35° to 40° and belong to the group of the 

 Dryden Flags and Shales. They consist of gi'eenish shales interbedded 

 with soft, micaceous, flaggy sandstones, which contain in their lower 

 part thin bands of tuff. Near the top of the section, bands of chert 

 often sandy and nodular, sometimes more massive, are intercalated with 

 these beds; one, containing plant remains, reaches a thickness of 2 feet, 

 3 inches. Beyond this point to the south-east excavations were made 

 in the bank, but they failed to reach solid rock. 



III. Conclusions. 



From the evidence obtained in the course of these excavations, the 

 Committee have drawn the following conclusions: — • 



(1) The plant-bearing cherts found in the trenches are interbedded 

 with the Dryden Flags and Shales, and are therefore of Old Eed Sand- 

 stone age. 



(2) The plant-bearing cherts exposed in the roadside section (fig. 3) 

 are also interbedded with Dryden Flags and Shales. The band is 

 probably the stratigraphical equivalent of the chert occurring in the 

 trenches to the east. It contains the same plant (Rhynia), and rests 

 on a similar bed of white clay. 



(3) The strata exposed in the roadside section between the diorite 

 on the west and the Dryden Flags on the east (the Craigbeg Series or 

 the ' Older Series ' of Dr. Mackie) lie between two faults, each of them 

 having a downthrow to the east. Owing to the intense silicification 

 which most of the rocks have undergone, their lithological characters 

 differ considerably from those of the normal Old Red Sandstone strata 

 of the Rhynie outlier. They may nevertheless be of Old Red Sand- 

 stone age. The precise stratigraphical horizon of these rocks has not 

 been definitely determined. 



' It is probable that there may be more than one fault on the west side of 

 the Rhynie outlier. 



