None the less, a broad classification is possible on chemical grounds, and the _acid_, _intermediate_, _basic_, and _ultrabasic_ grouping adopted by Judd has been found of great convenience. Among acid rocks we have _granite_ as the coarsely crystalline type, with potassium felspars prevalent and the excess of silica manifest as quartz. The finer grained and sometimes compact types are the _eurites_, _quartz-felsites_, or _quartz-porphyries_. When the rock contains more or less residual glass, we have what are now known as _rhyolites_, of which ordinary _obsidian_ is the most glassy representative. The opposite types, those of the basic group, include, at the coarsely crystalline end, _gabbro_ and _basic diorite_; the finely crystalline forms are styled _dolerites_, and those with a trace of glass, or at any rate very fine-grained and compact, are _basalts_. Glassy types are naturally rare in this group, owing to the unsuitable chemical composition. Between granite and gabbro lie various rocks of intermediate composition, some of them rich in soda rather than in potash. _Syenite_, _granodiorite_, and the diorites with a prevalence of soda over lime, are coarsely crystalline types. Compact types of these of course occur. It will be sufficient, however, here to name the forms with traces of residual glass, which range from _trachyte_, the type rich in potash, to _andesite_, which connects them with basalt, in a series where lime ultimately predominates over soda. In the ultrabasic group are a number of exceptional types. Olivine often becomes an important constituent, and the rocks then decompose into the soft green or reddish masses known as _serpentine_--or, more properly, _serpentine-rock_. Igneous rocks, owing to their range of mineral composition and of structure, combined with their general hardness, lend themselves to various economic purposes. While the granites, resisting atmospheric attack admirably in a polished state, provide our handsomest building-stones, dolerites and fine-grained diorites, which owe their toughness largely to the interlocked relations of their constituent minerals, serve as our most satisfactory road-metals. THE SCENERY OF IGNEOUS ROCKS Volcanic landscapes, where activity is very recent or still in progress, present a number of characteristic surface-forms. The cones that have accumulated round the vents surpass all other hills in regularity of outline, and the crater in the summit is often relatively large. Lava-cones may be steep-sided bosses when formed of protrusions of viscid rocks rich in silica, like the remarkable domes in the north of Bohemia, or they may present very gentle slopes where fluid basic lavas have been extruded. Tuff-cones are liable to be breached on one side, owing to the outflow of lava which the crater-wall could not sustain, and they then assume the form of a mountain in which glacial influences have hollowed out a cirque.