266 



HARD WICKE 'S SCTENCE-G OSSIP. 



In No. I there are five cases in which the whole 

 liquid-cavity is 6J times the size of the vacuity, one 

 case in which it is six times the size, and two cases in 

 which it is only 5J times. The mean '162 is pro- 

 bably not far from the truth. 



It maybe difficult to realize the size of these liquid- 

 cavities and their contained bubbles. Those reliable 

 for measurement are extremely small, sometimes less 

 ^th of an inch in diameter. Much larger 



than T 



cavities generally occur in plenty ; but these seldom 

 exceed the -j-oWth of an inc h i n length, and in them 

 the bubbles either have no movement, or but a very 

 slight or sluggish one. In fact, such is the minuteness 

 of these cavities and their number, in many cases, 

 that more than a thousand million might be contained 

 easily within a cubic inch of quartz, and sometimes 

 the contained water must make up at least 5 per cent, 

 of the volume of the containing quartz. In some 

 cases the liquid-cavities are much arranged along 

 lines, as in the quartz crystals occurring in the 

 ArmbothDyke. Occasionally, however, liquid-cavi- 

 ties are met with in quartz crystals of veiy considerable 

 size, such that the movement of the bubble can even 

 be recognized by the naked eye. Having accumu- 

 lated a sufficient number of reliable measurements, 

 and struck the mean for any one rock, such as the 

 Skiddaw granite, the calculation of the pressure 

 under which the granite was formed is proceeded upon 

 by mathematical formulae furnished by Mr. Sorby's 

 investigations, the temperature of a dull red heat 

 visible in the dark (68o° F.) being assumed as the 

 probable temperature of consolidation. The result, 

 in the case of the Skiddaw granite, is that a pressure 

 equivalent to 52,000 ft. of rock must have been 

 necessary to compress the liquid so that it would fill 

 the cavities at a temperature of dull red heat. 



The next question bearing upon this result is one 

 for the field-geologist alone to determine. What is 

 the greatest thickness of rock which can have been 

 removed from over the mass of the Skiddaw granite 

 as we now see it exposed ? Geological investigation 

 cf the whole district leads me to infer that at one 

 time the granite must have been covered by some 

 30,000 feet of rock — including Skiddaw slates, vol- 

 canic series, and Upper Silurians. But we have seen 

 that the calculated pressure is equivalent to 52,000 ft., 

 hence the pressure to which the consolidating granite 

 (and therefore quartz) was subject was greater than 

 what could be due to the mere weight of overlying 

 rocks. How, then, was this pressure applied ? We 

 have seen that the condensed liquid confined within 

 the quartz has remained as a registering thermometer, 

 to show the existence and amount of the pressure, 

 and geological examination of the district further 

 informs us that the overlying 30,000 ft. of rock was 

 in some way elevated and contorted, — being also 

 slowly removed by denudation. Thus we have 

 physical evidence, from two different sources, of the 

 existence of great pressure exerted upon the granite- 



forming mass ; evidence derived from the liquid- 

 cavities and their contained bubbles, and evidence in 

 the rocks overlying the granite of such pressure having 

 produced elevation, contortion, cleavage, and general 

 metamorphism. It is further interesting to notice 

 what would be the probable heat at a depth of 

 30,000 ft. according to our estimates of known 

 increase on descending through the crust of our globe. 

 If we take that increase as 1° F. for every 49 ft. 

 (Mr. R. W. Fox, Brit. Assoc. Report, 1857, p. 91), 

 we find that at a depth of 30,900 ft. the temperature 

 would be 360 C. (680 F.), or that of a dull red heat 

 visible in the dark, and just the temperature at which 

 we were supposing the granite was formed. Thus, 

 to sum up our results in the case of the Skiddaw 

 granite : — 



1st. It is probable from geological evidence that 

 this granite was consolidated at a depth of about 

 30,000 ft. 



2nd. An increase of i° for every 49 ft. on descend- 

 ing gives a temperature of 360 C. (680 F.) at a 

 depth of 30,900 ft. 



3rd. Microscopic evidence, deduced from examina- 

 tion of the liquid- cavities in the quartz, gives a 

 calculated pressure under which the quartz was con- 

 solidated at a temperature of 360 C. (680 F.), 

 equivalent to 52,000 ft. of rock. 



4th. As the calculated pressure thus far exceeds 

 (by 22,000 ft.) that due to the estimated thickness of 

 overlying beds, or, what is the same thing, to the 

 estimated depth at which the granite formation took 

 place, it follows that there must have been a great 

 amount of available pressure to be exerted upon the 

 rocky crust around, and hence we find evidences of 

 folding and contortion of the rocks, and of their up- 

 heaval and general metamorphism around the granitic 

 area. 



The case of the Skiddaw granite will suffice as an 

 example of this mode of treatment. I have elsewhere 

 {Quart. Joum. Geol. Soc, vol. xxxi., p. 568) treated 

 all the other granitoid rocks of the Lake District in 

 the same way, and with very similar general results. 

 There are yet, however, one or two points I should 

 like to call attention to. 



The first of these~is the general absence among the 

 liquid-cavities in the quartz of Lake District rocks of 

 crystals of various salts, such as have been found in 

 plenty in some other districts. Are we to infer from 

 this that the liquid enclosed at the period of the for- 

 mation of the granites was less saline than in some 

 other cases ? Or is impossible that a more extended 

 investigation would make it appear otherwise ? 

 Another thing worth noting is the general absence, 

 as far as I have been able'to judge, of cavities in the 

 quartz containing, not water, but liquid carbonic 

 acid, which in some other rocks and other districts 

 are not infrequent. 



While thus calling attention to various points con- 

 nected with the formation of quartz-be aring rocks of 



