242 



HARDWICK&S SCIENCE-GOSSIP. 



have the astounding fact revealed to us, that in every 

 small fragment of opaque quartz there are thousands 

 upon thousands of these minute liquid parcels shut 

 up within it, and each containing a vacuous bubble. 

 Again, we ask, what can this mean ? Can it be that 

 water has had so large a part to play in the formation 

 of this hard mineral quartz, that granite, which we 

 used to look upon as a molten rock formed in the 

 bowels of the earth, can yet have been formed largely 

 in connection with water ? for the quartz combining 

 with felspar and mica to form granite is literally full 

 of these minute water-cavities. Let us see how this 

 can be in the case of granite, and then other quartz 

 occurrences may prove easier to understand. Mr. 

 Sorby has made many and delicate researches into 

 this subject, and he has found that on subjecting the 

 liquid cavities to heat, first, any minute crystals of 

 salt contained in them are slowly dissolved ; and 

 second, that when the heat is made sufficiently ex- 

 treme the minute bubble becomes smaller and smaller 

 until finally the liquid fills the whole cavity, the 

 bubble completely disappearing. Now it is evident 

 that these little parcels of water must have been 

 enclosed within the quartz on its solidification, when 

 the granite was being formed deep beneath the surface, 

 for all geological evidence goes to show that granite 

 is deep-seated in its formation ; although of various 

 ages, yet no other rock is ever found beneath it. 

 Therefore, it is further evident that when first en- 

 closed the water must have been in a highly heated 

 condition, and unable to attain the state of vapour 

 by reason of the great pressure to which it was sub- 

 ject, so that we may regard the liquid now contained 

 in the quartz as part of the original mother-liquor, if 

 I may use the term, which, together with a heated 

 condition of other minerals, and great pressure, made 

 up the conditions under which the granite was formed- 

 In the formation of the granite the two minerals 

 mica and felspar were the first to crystallize, impress- 

 ing their form upon the semi-molten or semi-fluid 

 quartz, which solidifying last, wrapt up within itsel^ 

 large quantities of the highly-heated water in the 

 form of minute liquid parcels or cavities. Subse- 

 quently, on cooling of the mass, removal of pressure, 

 or both, the water would cool, but being closely 

 sealed up in glassy (quartzy) cavities or minute bottles, 

 it could not escape in the state of vapour any more 

 than at the first, but in its cooled condition would 

 occupy a slightly less space, in other words, the con- 

 traction would leave a minute bubble or empty space 

 (called for convenience' sake, a vacuity), which 

 vacuity you will see must have a size proportionate, 

 first, to the size of the liquid parcel ; second, to the 

 heat and pressure under which the fluid was origin- 

 ally confined. Hence, if it is found that in a number 

 of cases the relative size of vacuities and liquid- 

 cavities is the same, these relative dimensions give us 

 clue to the amount of combined heat and pressure 

 under which the granite (or, to speak more accurately, 



the quartz of the granite) was formed. Here, then, is 

 an interesting problem ; but at the outset it appears 

 that to find out the exact amount of either the heat 

 or the pressure, it is necessary to know that of one of 

 these elements ; thus, if we knew the pressure and 

 observed the relative size of vacuity (or bubble) to 

 liquid-cavity, we should have the two elements from 

 which to calculate the heat ; or, if we could form a 

 tolerably accurate estimate of the heat, and made 

 observations on the relative size of vacuity and liquid- 

 cavity, these elements'would allow us to form some 

 fairly correct idea of the pressure under which the 

 rock was formed. 



Now, by an elaborate series of investigations, Mr. 

 Sorby has shown it to be very probable that the heat 

 was not much greater than 360° C. (680° Fah.), or 

 not more than a dull red heat visible in the dark. 

 We have already seen how the water in many cases 

 contained, when hot, more mineral salts dissolved in 

 it than could be so contained when cold ; hence the 

 small crystals of salt sometimes found in the liquid- 

 cavities ; but Sorby has also called attention to the 

 fact, that crystals of the mineral called Schorl (a 

 variety of hornblende) are frequently enclosed in the 

 liquid-cavities, and that these would be fused at any 

 temperature greater than a dull red heat ; and from 

 this and other facts he has fixed on the temperature 

 of 360 C. (6So° F.) as the probable extreme at which 

 the quartz of granites was formed. Calculating on 

 the basis of these elements, and having made a large 

 number of observations upon the relative size of the 

 vacuities and liquid-cavities in the quartz of the 

 Cornish granites, Mr. Sorby arrived at the conclusion 

 that the mean of the pressure expressed in feet of 

 rock under which the Cornish granites were con- 

 solidated was 50,000 ft. (this not necessarily the 

 actual depth at which formed). 



( To be continued. ) 



THE CRITICAL BOTANIST. 



n^HE educational value of "natural" or biological 

 J- science, as distinguished from "physical" or 

 "experimental" study, depends on the stress laid, 

 not only on mere observation, but on observation at 

 once careful and minute. Experiment is being intro- 

 duced with much good result into the biological 

 sciences. It teaches handiness or skilful manipula- 

 tion, and exactitude in recording results. The 

 use of quantitative analyses and statistics has 

 also a great value, not only to science, but also 

 to education. Still, critical observation is the chief 

 educational raison d'etre of biology, and especially 

 of botany, to the tyro. In proportion as it is care- 

 ful and minute, will this observation be valuable 

 to the observer. Its value to science is really a 

 secondary matter, and will come unexpectedly, being 

 dependent, not only on care and minuteness, but also 

 on what may be termed "rationality." By this, I 



