78 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
crust, appear at the surface in rounded, elliptical or irregularly -shaped 
eminences, these are called Bosses (Stocks, Culots). 
Further contrasts between the superficial and subterranean consolidation 
of molten material are to be found in the respective textures and minute 
structures of the rocks. The deep-seated intrusions are commonly charac- 
terized by a general and markedly greater coarseness of crystallization than 
is possessed by lavas poured out at the surface. This difference of texture, 
obviously in great measure the result of slower cooling, shows itself in acid, 
intermediate, and basic magmas. A lava which at the surface has cooled as 
a fine-grained, compact black basalt, in which neither with the naked eye nor 
wdth the lens can the constituent minerals be distinctly determined, may 
conceivably be represented at the roots of its parent volcano by a coarse- 
textured gabbro, in which the felspars and ])yroxenes may have growm into 
crystals or crystalline aggregates an inch or more in length. Mr. Iddings 
has pointed out that the various porphyrites which form the dykes and 
sills of Electric Beak are connected witli a central boss of coarsely crystalline 
diorite.’- Examples of the same relation from different volcanic centres in 
Britain will be cited in later chapters. 
This greater coarseness of texture is shown by microscopic examination 
to be accompanied by other notable differences. In particular, the glassy 
residuum, or its devitrified representatives, which may he so frerpiently 
detected among the crystals of outflowing lavas, is less often traceable in the 
body of subterreanean intrusive rocks, though it may sometimes be noticed 
at their outer margins where they have been rapidly chilled by contact wdth 
the cool upper part of the crust into which they have been impelled. 
Various minerals, tlie constituents of wdiich exist in the original magma, 
but wdiich may be hardly or not all recognisable in the superficial lavas, 
have had leisure to crystallize out in the deep-seated intrusions and appear 
sometimes among the components of the general body of the rock, or as well- 
terminated crystals in its drusy cavities. 
Considerable though the variations may be betw’een the petrographical 
characters of the intrusive and extrusive rocks of a given district and of the 
same eruptive period, they appear generally to lie within such limits as to 
suggest a genetic relation between the whole series. Conditions of tempera- 
ture and pressure, and the retention or escape of the absorbed vapours which 
play so large a part in volcanic activity, must exercise great influence on 
the crystallization of constituent minerals, and on the consolidation and 
ultimate texture of the rocks. Slow cooling under great pressure and with 
the mineralizing vapours still largely retained seems to be pre-eminently 
favourable for the production of a holocrystalline texture in deep-seated 
portions of the magma, while rapid cooling under merely atmospheric 
pressure and with a continuous disengagement of vapours, appears to be 
required for the finer grain, more glassy structure, and more vesicular 
character of lavas poured out at the surface. 
Besides these differences, however, there is evidence of a migration of 
' 12th Ann. Jtep. U.S. Ocol. Survey (1890-91), p. 59.'). 
