scoriaceous or cinder-like, and the under portions likewise frequently exhibit a similar appearance. It is generally most solid towards the centre of the bed. The vesicles, or pores, in the upper and lower portions are often flattened, and are frequently filled with mineral matter. Sometimes these cavities may have been filled at the time the rock was being erupted, but in most cases the mineral matter would appear to have been introduced subsequently by the action of water percolating through the rock. Occasionally we meet with igneous rocks which are more or less vesicular and amygdaloidal throughout their entire mass. Others, again, often shew no vesicular structure, but are homogeneous from top to bottom. The texture is also very variable, and this even in the same rock-mass; some portions being compact or fine-grained, and others coarsely crystalline. As a rule the rock is most crystalline towards the centre, and gets finer-grained as the top and bottom of the bed are approached. Not unfrequently, however, an igneous rock will preserve the same texture throughout. The jointing is also highly irregular as a rule. But in many cases, especially when the rock is fine-grained, the jointing is very regular. The basaltic columns of the Giants' Causeway and the Isle of Staffa are well-known examples of such regularly jointed masses. Igneous rocks frequently decompose into a loose earthy mass (_wacké_), and this is most markedly the case with those belonging to the basic group. [Illustration: Fig. 20.--Contemporaneous Igneous Rock.] 53. Contemporaneous igneous rocks are frequently associated with more or less regular beds of _breccia_, _conglomerate_, _ash_, _tuff_, &c. These are evidently the loose volcanic ejectamenta which accompanied former eruptions of lava, and have been arranged by the action of water. Beds of such materials, however, frequently occur without any accompanying lava-form rocks. Nor are they always arranged in bedded masses. They sometimes appear filling vertical pipes which seem to have been the funnels of old volcanoes. The following section exhibits the general appearance of one of these volcanic _necks_. They are very common in some parts of Scotland, as in Ayrshire, and are frequently ranged along the line of a fault in the strata. Fig. 21 shews such a neck of ejectamenta, made up of fragments of various kinds of rock, such as sandstone, shale, limestone, coal, &c., sometimes without any admixture of igneous rocks. The strata through which the pipe has been pierced usually dip in towards the latter, and at their junction with the coarse agglomerate often shew marks of the action of heat, coal-seams having sometimes been 'burned' useless for a number of yards away from the 'neck.' [Illustration: Fig. 21.--Neck filled with Volcanic Agglomerate.]