CiENERAL INFRRENCES. 85 



These instantiw crucis may be multiplied tind extended to 

 every sedimentary deposit in which fossil trees are found, and 

 an estimate derived of the formative process of the rocky 

 mass, layer after la3^er, from the duration of time that a tree, 

 standing in water, was able to preserve its structure and 

 resist the destroying action of the elements. Look at any of 

 the nearest river embankments, margined b}^ trees, during the 

 season of floodings or overflows. Observe the oak, the hardest 

 specimen of our forests, engulfed in the waters, and then con- 

 sider the plain question. How many years, or months, or days, 

 will that noble stem and those giant branches be able to bear 

 themselves aloft, and laterally, or uprightly 1 or how long be 

 able to contend with the currents and other wasting agencies, 

 atmospheric and otherwise, by which they are assailed 1 Cer- 

 tainly not tens nor twenties — most assuredly not hundreds of 

 years — far less the thousands registered in the calendars of the 

 geologist ! If the silting process is to be gauged by the 

 enduring powers of wood immersed in water, it is not perhaps 

 too much to say, that all the 3^ellow sandstone rock of Dura 

 Den might have been collected, and piled bed upon bed, in a 

 shorter period than has been occupied in eroding and scooping 

 out the ravine itself b}" the action of the stream on the now 

 consolidated mass. 



We have a remarkable instance of rapid formation, by sedi- 

 mentary and volcanic processes, in the island of Mull, where 

 huge mountains of basalt have been formed and upheaved 

 within comparatively recent times. The headland of Ardtun 

 shows in a vertical cliff of several hundred feet in height, 

 alternating beds of basalt, tuff, j)umice or scorise, and ligneous 

 mud, all indicating that the island and district were the theatres 

 of great volcanic action during the tertiary age. This inter- 

 esting discovery was made by the Duke of Argyll, who, in a 

 paper published in the Geological Quarterly Journal for May 

 1851, adduces clear and satisfactory evidence of vast sub- 

 terranean operations in Mull and the other Avcstern isles, and 



