But in the majority of the more solidly-built composite volcanoes no such liquid reservoir can be formed within the volcanic cone itself. Under these circumstances, the lavas, especially those of more liquid character, tend to force passages for themselves among the rocks through which they are extruded. Wherever a weak point exists, there such lavas will find their way, and as the planes of stratification in sedimentary rocks constitute such weak places, we constantly find sheets of lava thus inserted between beds of aqueous origin. The areas over which these intrusive sheets of rock sometimes extend may be very great, but the more fusible, basic lavas (basalt, &c.) usually form much more widely-spreading sheets than the less fusible, acid lavas. In some cases these great intrusive sheets are found extending to a distance of twenty or thirty miles from the centre at which they were ejected, and they often follow the bedding of the strata with which they are intercalated in so regular a manner, that it is difficult for an observer to believe at first sight that they can have been formed in the way which we have described. A closer examination will generally reveal the fact that while these intrusive lava-sheets retain their parallelism with the strata among which they have been intruded, over considerable areas, yet they sometimes break across, or send offshoots into them, as shown in fig. 56. In all cases, too, the rocks lying above and below such sheets will be found to be more or less baked and altered, and this affords a very convincing evidence of the intrusion of the igneous mass between the strata so altered. [Illustration: Fig. 56.--Sheets of Igneous Rock (Basalt) intruded between beds of sandstone, clay, and limestone. (Island of Skye.)]