Chap. I] I 
STRUCTURES OF LAVAS 
movement of a flowing current of basalt.^ The great heat of tlie lava 
frequently induced considerable alteration upon the underlying rocks, 
^iiduration is the most common result, often accompanied with a reddening 
the altered suljstance. Occasionally a beautifully prismatic structure has 
^een developed in the soft material immediately beneath a basalt, as in 
ferruginous clay near the village of Esplot in the \ elay, in which the close- 
®6t columns are 50 centimetres long and 4 to 5 centimetres in diameter, 
tlhauges of this nature, however, are more frequent among sills than among 
Superficial lavas. Many examples of them may be gathered from the 
Scottish Carboniferous districts. 
Variations of structure in single lava-sheets. — From what has 
said above in regard to certain kinds of flow-structure among basic 
^ouks, it will be evident that some considerable range of chemical, but more 
Particularly of mineralogical, composition may be sometimes observed even 
within the same sheet of lava. Such diflerences, it is true, are more frequent 
U'uong intrusive rocks, especially thick sills and large bosses, tut thev 
^uve been met with in so many instances among superficial lavas as to show 
that they are the results of some general law, which probably has a wide 
‘Application among the surface-products of volcanic action. Scrope expressed 
the opinion that in the focus of a volcano there may be a kind of filtration 
‘Af the constituents of a molten mass, the heavier minerals sinking through 
the lighter, so that the upper portions of the mass will become more felspatlnc 
^'Ud the lower parts more augitic and ferruginous.® 
Leopold von Ihieh found that in some of the highly glassy lav as of the 
Lanary Islands the felspar increases towards the liottoni of the mass, becom- 
'AAg so abundant as almost to exclude the matrix, and giving rise to a com- 
pound that might be mistaken for a primitive rock.A 
Ilarwin oliserved that in a grey basalt filling up the hollow of an old 
crater in dames Island, one of the Clalapagos group, the felspar crystals 
^'ccanre much more abundant in the lower scoriaceous part, and he discussed 
^'Ae question of the descent of crystals by virtue of their specific gravuty 
through a still molten lavm.® 
Mr. Clarence King during a visit to Hawaii found that in every case 
"'here he broke newly-congealed streamlets of lava, “ the bottom of the flow 
""us thickly crowded with triclinic felspars and augites, while the whole 
"Pper part of the streain was of nearly pure isotropic and acid glass.” This 
®Aihject will be again referred to when we come to discuss the characters of 
Autrusive sills and bosses, for it is among them that the most marked petro- 
"Aaphical variations may be observed. Examples will be cited both from 
intrusive and extrusive volcanic groups of Ilritain. 
Volcanic cycles. — Closely related to the problem of the range of struc- 
1 M. Boule, BvXl. Cart. CM. France., No. 28, tom. iv. (1892), j). 235. 
- M. Boule. Op. cit. p. 234. * Volcanoes, p. 125. 
Description Fhysi<iue des Isles Ca'narics (1836), p. 190. 
Geological Observations mi Volcanic Islaiuls (1844), p. 117. 
“ U.S. Gcol. E.rploralion of the Fortieth Parallel, vol. i. fl878), p. 716. 
