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of about 3 (excluding the vitreous rocks), the latter of about 2*8. The 

 basic rocks shade into the intermediate rocks by the most insensible 

 gradations, and considerable difficulty is experienced in deciding where 

 the line of separation should be drawn. We shall have a practical illustration 

 of this in dealing with the Tertiary dykes of the North of England 

 and South of Scotland. The rocks of many of these dykes contain from 

 55 to 58 p.c. of silica, and so far as composition is concerned might, 

 therefore, be considered with more appropriateness in the next chapter. 

 We shall treat of them here, and indicate their special characters by speak- 

 ing of them as andesitic dolerites and basalts. 



Rocks of the dolerite group exhibit a wide range of variability so far 

 as the size of the individual constituents is concerned. The term dolerite 

 properly belongs only to the distinctly crystalline rocks. The compact 

 varieties are usually termed basalt. Individual masses of dolerite usually 

 show a considerable amount of uniformity in composition and texture. 

 Sometimes, however, we find contemporaneous veins, nodules, and irregular 

 patches differing from the main mass. 



The occurrence of glass or other kinds of amorphous interstitial matter 

 is an important feature of the rocks now under consideration. By an in- 

 crease in the amount of glass and by a diminution in the amount of the 

 crystalline matter the holocrystalline dolerites and basalts pass through 

 the magma-basalts into tachylyte or basalt-glass. It has been proposed that 

 the presence or absence of glass should be made the ground of distinction 

 between dolerite and basalt that the former term should be limited to the 

 holocrystalline rocks, and the latter to rocks containing glass. The 

 principal objection to this is that it would involve a considerable departure 

 from the accepted use of the terms. Many distinctly crystalline rocks 

 would have to be termed basalts ; and many compact rocks would become 

 dolerites. If the presence of glass were connected with any important 

 textural, mineralogical or geological characteristics the departure from the 

 generally accepted use of the terms would of course be justified ; but such 

 does not appear to be the case. It seems better therefore to retain the 

 common use of the terms, for this roughly corresponds with broad 

 geological facts. The dolerites usually form intrusive sheets or dykes 

 and the basalts, lava-flows. 



Another very interesting feature in rocks of the dolerite family is the 

 fact that the order of consolidation of the two principal constituents, pla- 

 gioclase and augite, is not always the same. In some cases the felspar pre- 

 cedes the augite (ophitic dolerites), in others the two minerals crystallize 

 simultaneously (pegmatitic dolerites), and in others the augite precedes 

 the felspar. The pegmatitic structure in dolerites is, so far as we know, 

 extremely rare; but it is very interesting as furnishing an intermediate 

 sta^e between the ophitic dolerites and the dolerites with idiomorphic augite. 



The presence of porphyritic crystals gives rise to important varieties of 

 basic igneous rocks. In some we find felspar (usually bytownite and less 

 frequently labradorite) occurring as a porphyritic constituent ; in others 

 augite ; in others both augite and felspar. It is important to note that 



