part 4] THE PETROLOGY OF THE ARNAGE DISTRICT. 469 



the Ythan, and the ground around Glencroft north of that river. 

 A. large part of this ground is well exposed; especially is this the 

 case along the Ythan Valley proper. 



In the Ardlethen district we encounter again the Arnage Type 

 (cordierite-hiotite-quartz-oligoclase - rock) and the Kinharrachie 

 Type (hornblende-biotite-andesine-quartz-rock) ; but the most 

 important feature is the development of an acid facies of the 

 contaminated rocks — the Ardlethen Type. This acid phase 

 appears to pass, near Glencroft, into a rock of so normal a granitic 

 nature that at first it was mapped as a granite of an ancestry in 

 which contamination had played no part. In addition to these 

 points of petrogenetic value, there are at Ardlethen certain 

 structural features of interest. The development of the Arnage 

 and Kinharrachie T} r pes will be described first, and then the 

 granitic end-products of Ardlethen Type. 



Contaminated rocks of Arnage Type are found along the south- 

 ern margin of the Mass from Hillhead of Ardlethen to the Cat 

 Craigs north of Ythsie ; they occur also, among other places, in 

 the wood half a mile north of Ardlethen, near Ardlethen itself, 

 and a third of a mile north of Glencroft. In all cases they hold 

 abundant xenoliths, either of hornfelsed biotite-schist of the Ellon 

 Series, or of hornfelsed andalusite-schist of the Fyvie Series. 

 In slices, rocks of this type are like those already described from 

 Arnage. and are composed of quartz, oligoclase, biotite, and 

 cordierite, with often abundant green spinel and pink garnet. 

 Certain of the rocks of this class contain much soda-micro- 

 cline and orthoclase, and provide excellent transitions towards 

 the Ardlethen Type presently described. Others lack cordierite, 

 and afford a second transition to the acid end-product. As will be 

 noted later (see p. 470), the cordierite-bearing type appears to 

 pass into the granitic Ardlethen Type. 



Some interesting phenomena, seen in the wood nearly half a mile 

 north of Ardlethen, may here be noted. When this wood is 

 viewed from the north, say from a point near Glencroft, it is 

 observed that there is a conspicuous hump in the general even 

 sweep of trees. When investigated more closely, this hump is 

 seen to be due to a cake of quartzite floating, as it were, on the 

 contaminated magmatic rock around it. This quartzite is part 

 of the Fyvie Series, and forms a cake, 400 yards long, in Avhich the 

 bedding dips steeply westwards. With the quartzite is associated 

 a narrow belt of limestone, seen at Ardlethen Quarry, and in the 

 wood 6C0 yards north by west and 900 yards north of Ardlethen 

 (see fig. 5). There can be no doubt that these represent patches 

 of the sedimentary roof preserved upon the contaminated belt. It 

 is to be noted that this quartzite cake is veined by cordierite-bearing 

 contaminated rocks, which carry xenoliths of blue argillaceous 

 homfels. The inference can be drawn therefrom that the quartzite 

 cake owes its preservation to its greater resistance to the corrosive 

 action of the magma. What are best considered as further examples 

 of these roof-phenomena are seen 1000 yards and 1200 yards 



