Geography. 8 1 



there layers of stratification will be formed, whether 

 they fall in the sea or on land. It has been suggested 

 by Mr. Ward that some of this fine volcanic dust fell 

 into lakes that filled old craters or areas of subsidence 

 during periods of partial repose, and this seems highly 

 probable, for the finely divided matter is so beautifully 

 stratified, that these beds were, and still are by some, 

 mistaken for marine strata. 



When we consider the vast amount of these pro- 

 ducts of ancient volcanoes, there can be no doubt that, 

 rising from the sea, some of them must have rivalled 

 Etna in height, and others of the great active volcanoes 

 of the present day, and, as most volcanoes have a conical 

 form, we can easily fancy the magnificent cones of 

 those of Lower Silurian age. But that is all we know 

 respecting them, and whether or not they were clothed, 

 like Etna, with terrestrial vegetation, no man can tell. 

 It is hard to believe that they were utterly barren, but 

 as yet no trace of a flora has been found in Lower 

 Silurian strata. 



There is another point bearing on the physical geo- 

 graphy of the time that has sometimes crossed my mind 

 in connection with these island volcanoes, which is, that 

 we may, with some show of probability, surmise, that 

 then, as now, the prevalent winds of this region blew from 

 the west and southwest, for the following reason. In 

 Merionethshire and Caernarvonshire the various volcanic 

 products gradually thin out and disappear to the west, 

 between the ground south of the estuary of the Mawddach, 

 and the neighbourhood of Tremadoc on the north. As we 

 pass round the large crescent- shaped masses of lavas and 

 ashes it becomes evident as a rule that the ashy series of 

 beds show a tendency to thicken more and more in an 

 easterly direction for a space, and finally to decrease in 



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