During the course of palaeontological work in the North Wales coalfield, shining discoidal bodies were noticed on the bedding planes of various shales. They were found to be identical with objects described as Guilielmites by Geinitz (4) in 1858. The literature concerning them is reviewed below, and a new theory of their origin is advanced.

The four genera discussed below are mentioned in almost every paper, memoir, and monograph dealing with the Lower Kimeridge Clay in this country and abroad. The first three were given currency in the literature of the Upper Jurassic when species belonging to them were chosen as index-fossils for the three lowest zones of the Kimeridge Clay by Salfeld (1913–14). Ataxioceras, though not comprising any zonal index fossils, is almost as important and has been repeatedly cited for 56 years from all parts of the world.

In 1836 Rose (Phil. Mag., viii, p. 30) described a series of marine deposits in the Nar Valley, associated with the brick clay of that neighbourhood. Later work revealed that these beds were widely distributed to the south-east and east of King’s Lynn, and were to be correlated with very similar gravels at March and Whittlesey. Further correlation outside this area is uncertain, but following Boswell’s scheme (1932, pp. 72, 75) the March-Nar marine episode may be classed with the well-known estuarine sections at Kirmington, Lines.: moreover, he refers all gravels containing Corbicula (including the Barrington gravels) to this period. The same author regards the uppermost boulder clay at Kirmington as of Upper Purple age, and “apparently not the equivalent…of the Brown boulder clay of Hunstanton”.

The present paper deals with a number of intrusions lying within a north-easterly oriented belt of country, 4 miles wide and 12 miles long, running from a point 1½ miles west-south-west of the Brack, through the Arrochar mountains and across the head of Loch Lomond, to the upper reaches of Glen Gyle, 3 miles east of Ardlui (Text-fig. 1). Only one of these attains any considerable size. It lies north-west of the head of Loch Long and may be called the main Arrochar intrusion.