592 J. G. Goodchild — Deutozoic Rocks of North Britain. 



such of Dr. Traquair's papers as bear upon this part of the subject 

 is given in an appendix to the present papei'.^ 



It need hardly be stated here that the Scottish rocks of Devonian 

 age differ in almost every respect from the normal type. There 

 is no satisfactory reason for regarding any of them as of marine 

 origin; and, on the other hand, there is much to be said in support 

 of the view that they were all formed under continental conditions, 

 and under conditions of climate which, though doubtless varying 

 mucli from time to time, were yet, on the whole, characterised by an 

 annual rainfall decidedly below the average in amount. It is this 

 feature which has imparted a common character to the whole of 

 this series of rocks. 



We may now proceed to review such of the chief points of interest 

 connected with each of the Old Eed Sandstones as bear more or 

 less directly upon the chief matter to be discussed in this paper. 



The newest subdivision of the Old Red Sandstones is the well- 

 known Upper Old Eed. This, by the way, is usually red in colour, 

 and, also, it does consist mostly of sandstones. The remark is not 

 altogether uncalled for, as much of the remainder of the Old Red 

 is not red, and it does not, by any means, consist exclusively of 

 sandstones. The Upper Old Red was formed at a period when 

 the geographical conditions were slowly changing from those of 

 a continental nature to those of an insular type. The rocks are, 

 of course, extremely variable in thickness, because they were laid 

 down upon a very uneven surface of the old land. They are also, 

 but to a lesser extent, variable in mineral character. But where this 

 formation is most fully and typically developed its petrographical 

 characters may be described as referable to three types — (1) a basal 

 conglomerate, which graduates upward into (2) a variable series 

 of red sandstones, often full of desert-sand grains, and highly false- 

 bedded in places, like an old desert sand-dune. This part is mainly 

 red, with some poikilitic mottling and variegation. A few bands 

 of marl- occur here and there. Calcareous matter is usually con- 

 spicuous by its absence from this division. Above this group of red 

 rocks usually occurs (3) a higher subdivision, in which the sandstones 

 are not so deeply coloured, and in which even purplish bands begin 

 to occur. This upper group is generally characterised by the 

 occurrence in it of some precipitated carbonate of lime. In some 

 cases this compound occurs in the form of flakes, which obviously 

 I'epresent brokeu-up sheets of chemically-formed carbonate of lime ; 

 in other cases the calcareous matter has segregated into a nodular 

 form ; while in some few instances the calcareous matter may 

 occur as lenticular masses closely resembling some bedded limestones. 



^ In here referring to it, I may perhaps be permitted to direct the reader's special 

 attention to the beautiful and lifelike restorations of the fishes of the Orcadian 

 Rocks which Dr. Traquair has given in Harvie Brown's ' ' Vertebrate Fauna of the 

 Moray Firth." 



2 The word ' marl ' in the Xorth means any clay that readily crumbles on exposure 

 to the weather. 



