VOLCANIC ACTIVITY 207 
ing in a series of falls to the deeper river, of which 
it is a tributary, a few miles away. There are deep 
pools in which the Brown Trout lurk, sweeping 
shallows, rocky boulders partly high and dry, and 
bubbling cascades. In parts the stream has literally 
carved its way through solid voleanic rocks, which 
now stand cliff-like on either side ; in other places 
farther along it has washed away shales and lime- 
stones and opened out a wider course. For ages 
this stream has been cutting and carving its way ; 
this glen is due to its ceaseless activity. Below us 
is the stream ; between it and the footpath there is 
a bank of rich vegetation, and on our left is an 
artificially made millrace, whose waters turn a big . 
millwheel about a mile and a half away. é 
I want you to notice this cliff, part of which is 
weathering very rapidly. Here is what looks at 
first like a bed of sand ; itis quite yellow and gritty. 
But it is not sand by any means. On the reverse 
it is a stratum of volcanic ash about six feet thick. 
Above it, and jutting out from it, you see a bed of 
lava about three feet thick. This is solid and 
hard ; it does not weather down so rapidly as the 
ash. Again, above the lava is another thick bed 
of ash. The story of this cliff is fairly easily read. 
The igneous rocks about here are of Lower Carbon- 
iferous age, when there was great volcanic activity. 
The volcano from which this material came first 
belched forth big showers of ashes, which settled 
down to form the lower ash-bed ; then it sent out 
a big stream of lava which formed the lava-bed ; 
afterwards it sent out more and yet more ashes, 
which gradually accumulated and made the upper 
