G. Hiclding — Old Red Sandstone of Forfarshire. 401 



Prosen, about Shielhill Bridge. They are the highest beds which, to 

 my knowledge, the system presents in this district. 



The yolcanic rocks of the district do not call for description here. 

 It may suffice to remark that they are, with very rare exceptions, in 

 the form of contemporaneous interbedded sheets, and are merely a 

 continuation of the series of the Ochil Hills, where they have been 

 described in detail by Geikie and others (A. Geikie, 1900). 



PoSITIOlSr OP THE FoSSIL-BEAEING BeDS. 



Attention must now be drawn to a point, the importance of which 

 has not, I believe, been hitherto recognised. All the recorded fossils 

 from this district — and I suspect that the same applies to Perthshire — 

 are from a very limited series of horizons near the middle of the 

 Lower Old Eed System, the Upper Old Red being, of course, left out 

 of account. The Carmyllie series is the fossiliferous group par 

 excellence, while a few of the worked localities may lie in the 

 Cairnconnan series (e.g. Tilliewharaland quarry, Turin Hill), or the 

 top of the Dunnottar conglomerates. Odd fossils have occasionally 

 been obtained from other horizons, but they are quite a negligible 

 quantity. 



The palseontological evidence as to the age of this Lower Old Red 

 System, then, as here developed, can only be affirmed of its middle 

 beds ; to apply it to the whole system, as has been implicitly done, is 

 obviously unjustifiable. Geikie has stated that these rocks may reach 

 a maximum thickness of 20,000 feet, which cannot be any great 

 exaggeration. The Orcadian Old Red he estimates at 16,000 feet; 

 then follows a considerable unconformity not represented by any 

 deposits, in Scotland at least, and some 2,000 feet of Upper Old 

 Red above that. Add these up and we obtain the modest sum of 

 38,000 feet with a great unconformity, or certainly the equivalent 

 of over 40,000 feet of strata to represent the Old Red Sandstone ! 

 Small wonder Sir A. Geikie shrank from admitting such a thickness, 

 and turned to Austen's hypothesis of isolated lake-basins to account 

 for the difference of the ' Caledonian ' and ' Orcadian ' faunas, 

 without having to superpose the latter rocks on the former ! 



Unfortunately for this solution of the difficulty it is becoming more 

 and more obvious that the hypothesis is untenable. The objections 

 fall into two categories — physical and palaeontological. Those of the 

 former class were well stated by Macnair & Reid in 1896 (M. & R., 

 1896). At Towie, in Aberdeenshire, an outlier of 'Orcadian' Old 

 Red approaches within barely twenty-five miles of the main mass of 

 the Caledonian deposit; and this outlier, like that of Tomintoul 

 a little further west, is actually on the Grampian ridge which must 

 have separated the two Old Red basins, so that it is very improbable 

 that at the time when it was deposited the ridge was in any important 

 degree higher than at present. Bearing in mind this fact, it is only 

 necessary (as pointed out in the paper just referred to) to replace the 

 Caledonian deposits in their original position, as before they were 

 faulted down, to see that all the higher beds must pass over the 

 Highland ridge to join the Orcadian series. 



DECADE Y. — VOL. V. — NO. IX. 26 



