3?6 FOSSIL-BEARING ROCKS OF ASSYNT. 



(Ionian. It includes in the Erriboll area a lower division, the Arnaboll 

 series, well developed in Ben Arnaboll, which consists of grey grani- 

 toid felspathic gneiss, overlain by dark hornblendic gneiss and mica 

 gneiss, which present a striped aspect. The upper part of the Cale- 

 donian consists of the ordinary flaggy gneisses, exposed on Loch 

 Hope and Ben Hope. This Hope series preserves its character from 

 Loch Erriboll to Loch Broom, and eastward to Lairg in Sutherland 

 and Ben Wyvis in Koss. The upper gneiss is thin-bedded and 

 highly quartzose, sometimes passing into quartzose schist, and some- 

 times into felspathic gneiss. 



The Assynt Series. A third group is the Assynt series, which 

 now lies between the western and eastern gneiss. The rocks are 

 comparatively unaltered. The quartzites contain annelid remains, and 

 possibly plants. The lowest of these Assynt rocks are subdivided 

 into the Tomdon sandstone below and Ben More grit above. This 

 grit, composed mainly of quartz and gneiss, has the fragments, some- 

 times several inches in diameter, embedded in a chloritic ground mass, 

 and passes up into green grits and other fine sediments, sometimes 

 sandstone, sometimes shale. The Torridon rocks appear to be about 

 300 feet thick, and though the bottom conglomerate covers miles of 

 country, it is n<Dt more than 100 feet thick. 1 



Over the Torridon sandstone rocks are the quartzites and seamy 

 quartzite with felspar grains, covered by the Annelid-bearing quartzite, 

 or pipe rock, characterised by vertical burrows, which is seen between 

 Loch Broom and Loch More, and on Loch Erriboll. These quartzites 

 are about 300 feet thick. 



Next succeed the brown flags, fine-grained sandy rocks, some- 

 times argillaceous, sometimes dolomitic, and these rocks graduate 

 upward into the grit, which contains Salterella Maccullochii. Finally, 

 the Assynt series includes the dolomite, which in its lower part is 

 dark grey or black, and in its upper part nearly white. The dolomite 

 is about 300 feet thick. The total thickness of the Assynt series is 

 upwards of 1000 feet. 



All these rocks have been greatly contorted, so that the Assynt 

 series is doubled back upon itself all along Loch Erriboll in a com- 

 pressed synclinal fold, which brings the quartzite over the dolomite ; 

 and there is an overthrow of the eastern gneiss or Caledonian forma- 

 tion upon the Assynt series, and sometimes the Caledonian is brought 

 over the Assynt series by a reversed fault. On Loch Broom the 

 Assynt series dips to the north-east, while the Caledonian series dips 

 to the south-east. The Hebridean gneiss on Loch Broom is in con- 

 tact with and slightly overlies every member of the Assynt series, 

 and in its overthrow is sometimes accompanied by the Torridon sand- 

 stone. Dr. Calloway 2 regards the Caledonian gneiss as having been 

 deposited unconformably upon the Hebridean rocks ; but the relations 

 of the two gneisses have yet to be demonstrated. 



1 See Drawings of Scenery in a paper by Heddle, in Journal Mineral Soc., 

 1880. 



2 Q. J. G. S., vol. xxxix. p. 355. 



