1 86 THE COOLED SURFACE OF LAVA. 



quarter of an hour melted down a hill of volcanic matter 50 feet high 

 and carried it away. On the other hand, a stream from Kilauea in 

 1839, catching the branches of trees, scarcely scorched the bark, and 

 the lava hung from them in masses like stalactites ; while islets of 

 forest trees were swept along on the surface, without killing the 

 bamboos, and only partially injuring the foliage. 



Kate of Flow of Lava. The rate at which the lava flows is neces- 

 sarily variable, but it may be remarked that the stream from Etna in 

 1669 ran 13 miles in the first 20 days, while it occupied 23 days 

 in covering the last two miles. The fluid lavas of the Sandwich 

 Islands have a far greater velocity, Dana stating the average progress 

 of the stream of 1839 at 400 feet an hour. 



Aspect of Lava Fields. The island Hawaii exhibits in a striking 

 manner some of the varieties which fields of lava assume on cooling. 

 Large tracts consist of smooth solid lavas, with undulating ridges 

 and hillocks some 10 to 20 or 60 feet high, and with the surface 

 marked with folds and lines, such as may be seen on glassy slags. 

 When the elevations are broken through, they are seen to be hollow, 

 and due to the ascent of volumes of vapour, which have formed 

 subterranean caverns. Other regions are rough, and termed " clinker 

 fields." They are covered with angular blocks and rough slabs 

 of every possible size, lying in the utmost confusion. These fields 

 stretch for miles, and are characterised by a grey and black desolate 

 condition. The transition from one surface of lava to the other 

 is frequently abrupt. The clinker fields are supposed by Professor 

 Dana to result from a lava stream floating on its surface, the materials 

 of a stream which had cooled, in much the same way as ice is carried 

 by a river in spring ; and perhaps the fact that the clinker fields rise 

 20 or 30 feet above the smooth lavas favours this view. The fissures 

 in lava streams are often seen to have been filled after the crust 

 of the lava had cooled, so that miniature dykes are formed, and 

 raised a little above the surface. This aspect characterises the whole 

 of the southern and south-eastern part of the island, and is termed 

 by Dana "the wearying grandeur of desolation." The European 

 lavas are rarely smooth. They more frequently have an irregular 

 billowy appearance, because the hardened external film is long 

 carried on with the stream, and rolled over and over till the con- 

 solidated surface becomes extremely rugged. Professor Phillips 

 observed that the hardened crust of the Vesuvian lavas sometimes 

 forms tunnels, from which the lava may run out so as to leave an 

 arched roof, which, however, soon falls in as the lava cools. 



It seems probable that one cause of the fluidity of greatly heated 

 lava must be its freedom from dissolved water, since steam, in [ex- 

 panding, can only tend to arrest its movement. 



Surface of a Volcano. The form of the mountain thus built up 

 depends a good deal upon the nature of the ejected materials. Those 

 cones which are formed chiefly of ashes are often regular and conical ; 

 while those formed chiefly of lavas are frequently so rugged as not 

 to suggest at first sight a volcanic origin. The regularity, too, may 



