Trap-pebbles of the boulder--Thickness of the earth's crust unknown--Not of much consequence to the practical geologist--Interior of the earth in a highly heated condition--Proofs of this--Granite and hypogene rocks--Trap-rocks: their identity with lavas and ashes--Scenery of a trappean country--Subdivisions of the trap-rocks--Intrusive traps--Trap-dykes--Intrusive sheets--Salisbury Crags--Traps of the neighbourhood of Edinburgh--Amorphous masses--Contemporaneous trap-rocks of two kinds--Contemporaneous melted rocks--Tests for their age and origin--Examples from neighbourhood of Edinburgh--Tufas or volcanic ashes--Their structure and origin--Example of contemporaneous trap-rocks--Mode of interpreting them--Volcanoes of Carboniferous times--Conclusion, 235 THE STORY OF A BOULDER. CHAPTER I. Scene near Colinton in midsummer--A grey travelled Boulder--Its aspect and contents--Its story of the past. Three miles to the south-west of Edinburgh, and not many hundred yards from the sequestered village of Colinton, there is a ravine, overshaded by a thick growth of beech and elm, and traversed beneath by a stream, which, rising far away among the southern hills, winds through the rich champaign country of Mid-Lothian. It is, at all seasons of the year, one of the most picturesque nooks in the county. I have seen it in the depth of winter--the leafless boughs doddered and dripping, the rocks dank and bare save where half-hidden by the rotting herbage, and the stream, red and swollen, roaring angrily down the glen, while the families, located along its banks, fleeing in terror to the higher grounds, had left their cottages to the mercy of the torrent. The last time I visited the place was in the heart of June, and surely never did woodland scene appear more exquisitely beautiful. The beech trees were in full leaf, and shot their silvery boughs in slender arches athwart the dell, intertwining with the broader foliage and deeper green of the elm, and the still darker spray of the stately fir. The rocks on either side were tapestried with verdure; festoons of ivy, with here and there a thread of honey-suckle interwoven, hung gracefully from the cliffs overhead; each projecting ledge had its tuft of harebells, or speedwell, or dog-violets, with their blue flowers peeping out of the moss and lichens; the herb-robert trailed its red blossoms over crag and stone; the wood-sorrel nestled its bright leaves and pale flowerets among the gnarled roots of beech and elm; while high over all, alike on the rocks above and among the ferns below, towered the gently drooping stalks of the fox-glove. The stream, almost gone, scarcely broke the stillness with a low drowsy murmur, as it sauntered on among the _lapides adesos_ of its pebbly channel. Horace's beautiful lines found