J. W. JUDD ON THE SECONDAKY EOCKS OF SCOTLAND. 687 



to correlate with one another, to show that they constitute portions 

 isolated by denudation of a once widely spread formation which 

 attained to a very great thickness, certainly exceeding 1000 feet, and 

 to demonstrate that this formation rests upon Carboniferous, and over- 

 laps older strata, and is covered conformably by the strata of Infra- 

 lias, Lias, and Oolite age. Further, it has become manifest to me 

 during my study of the district, that the strata in question present the 

 closest agreement in physical characters with deposits on the eastern 

 side of Scotland which have been rendered famous by the contro- 

 versy concerning the vertebrate remains which they contain : I refer 

 to the Eeptiliferous Sandstone of Elgin and the accompanying Stot- 

 field rock. Under all the circumstances of the case we can have little 

 hesitation in referring all the strata in question, which have so wide 

 a distribution in Sutherland, Elgin, Koss, Argyllshire, and in a 

 number of the Hebridean islands, to that series of formations which 

 in our islands intervenes between the Carboniferous and Jurassic 

 systems, and for which the name of Poikilitic has been suggested by 

 Conybeare and revived by Huxley. The Poikilitic strata of the 

 Highlands present striking resemblances, in their physical characters, 

 their stratigraphical relations, and their fossil contents, to the beds 

 of the same age in the more southern districts of the British Islands. 



These Poikilitic Strata of the Western Highlands consist of a very 

 variable series of deposits — breccias and conglomerates alternating 

 with variegated sandstones, clays, and impure concretionary lime- 

 stones. The conglomerates are, in some instances, found to be com- 

 posed of perfectly well-rounded pebbles ; more frequently, however, 

 the fragments of which they are made up are subangular; and occa- 

 sionally they retain their unworn edges and angles, and the rock 

 thus passes into breccia. The associated sandstones are usually 

 coarse-grained, and show a tendency to become, in places, conglo- 

 meratic, while false-bedding, ripple-marked surfaces, and clay-galls 

 are phenomena very frequently exhibited by them. The clays, 

 which are often more or less sandy, are usually of red, green, or 

 mottled tints, and strikingly resemble the so-called " marls " of 

 the English Poikilitic. The limestones are usually very impure in 

 composition — sometimes argillaceous, at others sandy in character ; 

 and they are not unfrequently associated with, and pass into, a 

 rock of siliceous composition. 



The materials forming the breccias, conglomerates, and coarser 

 sandstones have evidently been derived directly from the rocks on 

 which the Poikilitic strata at the particular locality repose — usually 

 either the Torridon Sandstone or some member of the great gneissic 

 series of the Highlands. One feature which the Poikilitic conglo- 

 merates and sandstones all present, and which serves to distinguish 

 them from all other similar strata in the Highlands, is the abundance 

 of calcareous material which enters into their composition. Not 

 only do fragments of compact limestone (derived probably from the 

 Durness or other calcareous beds now wholly removed by denu- 

 dation) abound in these strata, but their particles are frequently 

 united by a cement of carbonate of lime, the sandstones thus 



Q. J. G. S. No. 135. 2 z 



