Vol. 69.] THE VOLCANIC ROCKS OF THE FORFARSHIRE COAST. 461 



forms the promontory that juts out eastwards on the south side of 

 the Bay. These lavas are succeeded by sandstones, all dipping 

 consistently south-eastwards. 



Broadly speaking-, therefore, the field-relations of these rocks are 

 quite simple. When, however, we descend to detail, the petrological 

 similarity of the different lava-flows, their lenticular character, as 

 also the lenticular character of the conglomerates and sandstones 

 which are found among them, make it very difficult to ascertain 

 the precise effects of the faults that frequently occur, and con- 

 sequently to obtain a correct idea as to the order in which the' lava- 

 flows succeeded one another. 



The excellence and accessibility of the cliff-sections between the 

 Bed Head and Lunan Bay has led to an attempt to investigate 

 their structure in greater detail than has been found possible in 

 dealing with the coast between Lunan Bay and Montrose. 



II. Field-Belations of the Bocks. 

 (a) Montrose to Lunan Bay. 



About a quarter of a mile south-west of Afontroseness Lighthouse, 

 & fault occurs, separating two types of lava which can be dis- 

 tinguished one from the other quite easily. 



North of the fault, and extending round the coast as far as 

 Perryden, the volcanic rocks nearly all contain numerous lath- 

 shaped felspar crystals, which are particularly obvious on weathered 

 surfaces. These rocks l may be termed enstatite-olivine- 

 basalts. In colour they vary considerably, black or grey-black 

 predominating, though some are greenish, purple, or brownish-red. 

 The lavas are individually of no great thickness, rarely exceeding 

 10 or 12 feet, and are bedded in a very irregular manner. At 

 the base of each sheet occurs a few inches of compact, almost flinty 

 rock, which is moulded upon the irregular surface of the rock 

 beneath. Above, the rock becomes obviously coarser in grain, and 

 this passes upwards into very slaggy purplish lava, usually full of 

 green amygdales, and much weathered. 



The amygdaloidal lava is generally much fissured, the fissures 

 and larger cavities being filled with hard, fine-grained, greenish 

 or brownish-red sandstone, which often penetrates into the more 

 compact lava below, where the fissures become less numerous and 

 thin out. The sediment frequently expands into sheets, usually not 

 more than a few feet across, at the surface of the lava. 



The regular recurrence of these characters, which are typical of 

 the lavas of Lower Old Bed Sandstone age, 2 suggests the gradual 

 accumulation of sheets of rock solidifying from the molten state 

 under conditions that allowed of the contemporaneous deposition 



1 Petrographical descriptions of these and other rocks named in this section 

 are given later in § III, pp. 472-75 ; § IV, pp. 475-77. 



2 Sir Archibald Geikie, ' Ancient Volcanoes of Great Britain ' vol. i (1897) 

 pp. 2G3-347. 



