60 GEOLOGY OF THE COMSTOOE LODE. 



centric belts of magnetite have been observed, separated by hornblende-sub- 

 stance. Much tlie finest instance is illustrated in Fig. 17, Plate III. Tliere 

 can be little doubt from direct observation on modern lavas that porphy- 

 ritical crystals are formed prior to eruption, and a tolerably large and ver)^ 

 sharply defined specimen, like that shown in the drawing, is not likely to be 

 an exception. At some time after it ceased to grow this crystal was broken ; 

 but the external black border was formed at a still later period, for it is as 

 heavy on the fractured surface as on the crystal faces. It is difficult to imagine 

 a mass of melted lava in a state of agitation sufiiciently violent to break crys- 

 tals suspended in the fluid magma, except during an actual eruption, and it 

 may be inferred with some probal;)ility that this was fractured in its passage to 

 the surface. If so, the external black border was probably formed as the rock 

 cooled after eruption. The inner belt of magnetite, on the other hand, indi- 

 cates a check in the growth of the crj'stal, and must have been formed long 

 before ejection. But it is impossible to suppose the temperature to vary greatly 

 in melted rock-masses, at the depth below the surface at which it is believed 

 that eruptions originate. The pressure upon subterranean fluid masses, 

 however, probably varies within very wide limits, and it is well known that 

 changes in pressure produce effects closely analogous to those caused b}' 

 variations in temperature It seems on the whole, therefore, most likely 

 that this hornblende grew to the limits of the inner black border under 

 conditions which were uniform, or perhaps varied uniformly; that a sudden 

 change in pressure equivalent to a diminution of temperature induced the 

 secretion of magnetite; that the conditions for hornblende secretion were 

 then reestablished, and continued till the time of the eruption, when the 

 crystal was fractured, and became surrounded by a second border during 

 the cooling process. Other large hornbleiides in the same slide also have 

 double black borders, though less synunetrically developed, but the smaller 

 hornblendes, though also of considerable size, and nianifestl}^ "crystals of 

 first consolidation," show only a single external belt of magnetite, as if tlieir 

 formation had begun only after the temporary change in pressure. If tlie 

 hypothetical history suggested is correct, it is probable that hornblende only 

 forms under conditions of pressure which have not yet been reproduced in 

 the attempt to crystallize the mineral artificially, and the comparative rarity 



