Vol. 69.] TOLCANIC ROCKS OF THE FORFARSHIRE COAST. 481 



At Dunninald Den, the fault-breccia consists of a broken-up mass 

 of volcanic rock and of the sediment usually found among the 

 lavas, the whole being consolidated by secondary silica. 



VI. Conclusions. 



The Lower Old Red Sandstone rocks in this area furnish 

 abundant evidence of a long-continued series of volcanic eruptions. 

 The products of the eruptions are lavas which appear to have been 

 poured out in water in which tine sediment was accumulating, the 

 lava being much fissured and the fissures and cavities in it being 

 filled with sediment. Lenticular beds of conglomerate resting upon 

 denuded surfaces of volcanic rocks indicate that the volcanic material 

 occasionally appeared above the water, although the most, prevalent 

 •conditions were those of subsidence. A rude pillow-structure has 

 been observed in the basal portion of some of the lava-flows. 

 No definite pyroclastic material has been found. 



The rocks are almost entirely olivine-basalts, though in a few of 

 them olivine is not present, its place being taken by rhombic 

 pyroxene. It is possible that some of these may be basic andesites, 

 but their felspars are as basic as those of the typical basalts, and 

 their structures also correspond with those of some of the types 

 rich in olivine. 



A few dykes of Lower Old lied Sandstone age occur, which are 

 less basic than any of the lavas. They may be termed porphyrites. 



Several of the fault-breccias have been cemented with silica 

 derived from the decomposition of the adjoining volcanic rocks, 

 and in consequence are better able to resist the action of tho 

 weather, thus producing features outwardly similar to dykes. 



The fine sediments associated with the lavas bear evident traces 

 •of the pouring over them of molten rock prior to their consolidation. 

 The effects of deformation due to mechanical stress acting on un- 

 consolidated sediments, mineralogical changes, and cavities (probably 

 •due to the boiling of water in the sediments or the bubbling of 

 .gases through them), the cavities being now occupied by secondary 

 minerals, have all been observed in these sedimentary rocks. 



Incidentally, the examination of the coast-sections has revealed 

 the presence, in the south-west of Lunan Lay, of Avhat is probably 

 a mass of Upper Old Red Sandstone resting unconformably upon 

 the lavas of Lower Old Red Sandstone age. 



My best thanks are due to Prof. Sir Thomas H. Holland, K.C.I.E., 

 for providing me with every facility for carrying out this piece of 

 work, and for his kind help and advice during its progress. I am 

 ■also deeply indepted to Dr. G. Hickling, who not only suggested 

 the work to me, and placed at my disposal his own intimate 

 knowledge of the locality, but has also assisted me throughout 

 with friendly advice and criticism. 



