1894. NOTES AND COMMENTS. 173 



of the lavas of the great basaltic plateaux of the Inner Hebrides, 

 a river flowed across the volcanic plain from the Western Highlands, 

 whence it carried large quantities of shingle. By successive violent 

 floods, these materials, together with the detritus of the lava-fields, were 

 strewn irregularly far and wide beyond the immediate channel of the 

 river. In the pools thus left behind, fine volcanic silt gathered and 

 entombed leaves and stems of the surrounding terrestrial vegetation. 

 But volcanic activity still continued, and though ashes, slags and 

 pumice were swept down, new eruptions took place by which masses 

 of rock, sometimes nine feet in diameter, were thrown out to a 

 distance of a mile or more, and fresh streams of lava were poured 

 out, completely burying the previous accumulations. Renewed river- 

 floods of gradually-lessening severity spread fine detritus over the 

 cooled sheets of basalt, and again these later fluviatile deposits were 

 entombed between fresh outbursts of lava. Perhaps no more striking 

 evidence can be elsewhere obtained of the condition of the land 

 surface over which, from many scattered vents, the materials of the 

 volcanic plateaux of the Inner Hebrides were slowly piled up." It is 

 interesting to find that in these far-distant Tertiary times the slope 

 of the drainage areas of the Western Highlands was the same as in 

 our own day. 



Landscape Marble or Cotham Stone. 



The curious deposit known as Cotham Stone has exercised the 

 ingenuity of many to find a cause for its formation. As early as 1754, 

 Edward Owen published a work entitled " Observations on the 

 Earths, Rocks, Stones and Minerals for some miles about Bristol, and 

 on the Nature of the Hot Well and the Virtues of the Waters," and 

 in this book he suggests that the peculiar arborescent structures of the 

 Cotham Stone were produced by the escape of imprisoned air, which 

 permitted the blacker portions of the mud to follow the bubbles, and 

 so get dispersed in tree-like patches. Others have thought that 

 gaseous emanations from the Avicula conlorta-shdiies had produced the 

 peculiar markings, and others again suggested that mineral infiltra- 

 tions would account for them. 



Mr. H. B. Woodward in 1893 (Mem. Geol. Surv. The Jurassic 

 Rocks, vol. iii., p. 31) gave it as his opinion that the "arborescent 

 markings were produced during the consolidation of the stone, and 

 more particularly by the shrinking of its upper portions. In this way, 

 and while the mud was still in a more or less pasty condition, one or 

 more of the dark films in the banded mass were disarranged and 

 dispersed in arborescent form in the slowly-setting rock." He 

 suggested also that pauses in deposition and exposure to the sun's rays 

 might have accounted for some of the structure, and concluded that 

 markings were due mainly to mechanical forces, though there were 

 evidences of chemical change. 



