BUFFALO SOCIETY OF NATURAL SCIENCES 1 27 



regions. In the northeastern portion of the Central Belt in the basin 

 of the Gala Water, the various Birkhill zones are separated from each 

 other by thick beds of grits, conglomerates, and greywackes. Even 

 the graptolites show the effects of the great inpouring of fresh water, 

 for not only are they rare, but those which are found are dwarfed 

 as would be expected of a fauna dwelling in brackish water. Such 

 features point beyond a doubt to the oscillatory conditions which 

 prevailed along the shoreline and just so far as those conditions can 

 be traced southward so far may we say the sea retreated in Llandovery 

 and Tarannon time. It is only along the south central portion of 

 the southern margin of the Central Belt that the highest Tarannon 

 rocks are found; their continental origin is undoubted. They are 

 unfossiliferous except for tracks and trails and they consist of grey, 

 green, and red shales with bands of conglomerates one or two feet 

 thick. All of these facts indicate a lithological replacement of marine 

 by terrestrial deposits along a northwest-southeast line. The faunal 

 replacement is equally striking. Along the northern border of the 

 Tarannon belt the coarse deposits contain no fossils, but tracks and 

 trails; when a few dark shale bands appear, they usually contain not 

 good zonal fossils but a mixed Llandovery and Tarannon fauna. 

 Monograptus exiguus is recognized as the lowest graptolite in the 

 Tarannon and yet it frequently occurs with Rastrites maximus and 

 Cllmacogr aphis normalis, the former of which is the zonal fossil for 

 the uppermost Birkhill, and both of which are typical Llandovery 

 forms. Towards the south, however, this interfingering and mingling 

 of faunas is no longer noticeable and the Tarannon passes into the 

 shaly, mudstone phase where zonal graptolites are well recognized, 

 though in the passage to the upper Tarannon the mud facies is again 

 replaced by conglomerates. The evidence supplied by the litho- 

 logical and faunal characteristics, each taken independently, points 

 conclusively to a replacing overlap and to the terrestrial origin of 

 the Tarannon. The facts may be set forth in a generalized section. 



(/pper ^//-/^/// z f''' 



Fig. 11. Ideal Section Showing Restoration of Conditions During 

 Llandovery-Tarannon Time in South Scotland 



