43 



is nearly out of the question. Then, we get sun-cracks — the 

 desiccation fissures produced by the partial drying of the surface 

 of sand flats while the water was low in the old lakes. Besides 

 this we get gas-bubbles here and there in some of the top beds of 

 the Penrith Sandstone — just such bubbles as are formed in the 

 mud of shallow ponds by the evolution of marsh gas. Lastly, the 

 Penrith Sandstone has yielded many impressions of the feet of the 

 strange animals that wandered over the half-consolidated silt of the 

 old sand flats. These footprints are especially interesting, because 

 they afford us the sole traces of groups of animals long since 

 extinct — animals whose very forms are quite unknown to us, and 

 whose nature can be inferred only from the resemblance between 

 their footprints and those of their — perhaps totally different — 

 representatives of the present day. 



There is yet another point connected with the Physical Geo- 

 graphy of this region that I cannot pass over without a brief notice. 

 When we trace the Penrith Sandstone across the north-western parts 

 of Dumfries, in the direction of Ayrshire, we find — so Professor 

 Geikie tells us — that bands of volcanic matter, marking the occur- 

 rence of intermittent showers of volcanic ashes, begin to be inter- 

 stratified along with Penrith Sandstone of the ordinary character, 

 and when we get fairly into Ayrshire, these beds of volcanic tuff" 

 and breccia are further accompanied by beds of rock representing 

 old sheets of lava. We know from this, that a volcano, probably a 

 group of volcanoes, was in full action in those old times at a point 

 almost visible from Nunnery Walks. On the Continent volcanic 

 action seems to have been equally rife at the same period. 



When it is considered that the Penrith Sandstone is almost the 

 sole representative in the British Isles of the rocks missing between 

 the Carboniferous Series and the Magnesian Limestone, the im- 

 portance of a careful study of all that is likely to throw any light 

 upon the history of these parts in those remote times, will be evident 

 to most of us. I shall therefore now proceed to lay before you 

 what evidence we have been able to collect regarding the climate 

 of that period. In ordinary questions of this kind the geologist at 

 once betakes himself to the study of the various organisms en- 



