Penny Blood: Serial Sensation and Working-Class Entertainment — page 142: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

. x.) DISTURBED TRACT. 125 » The function A oE , which we have provisionally adopted to express the friction at the depth a, would actually vanish at the bottom of the crust. This of course represents what could not really happen. It is however evident, that any softening of the lower parts would tend to raise the neutral zone to a higher posi- tion, and that the more complete the softening the higher it would be. We have here assumed an amount of softening which is too great, and find that the corresponding position of the neutral zone would be at the depth of 4 of the crust. On the other hand we have found that, if the crust were equally rigid through- out, 1t would be situated at the depth of 4 of the thickness. Comparing the two cases, we may fairly conclude that it will be in fact in a position intermediate between these two, and shall be probably justified in placing it at about 3 of the thickness. On different, and geological, grounds we have likewise the following considerations to help to guide us in affixing a limit below which we must place the neutral zone. It is well known that granite frequently occupies the central axis of mountain chains. Now Mr Sorby has taught us in his sec- tional address at the British Association, 1880," that granite has been usually formed in the presence of liquid water; although he speaks of a single instance in which the temperature at the commencement of crystallization may have been higher than that at which water can exist in the state of a liquid. The critical temperature for water is about 773°F. At the rate of increase of one degree F. for 51 feet, this temperature would be reached at the depth of about seven miles and at the rate of one degree for 60 feet at the depth of about eight and a half miles. We may therefore conclude, that rock, which was once at seven or eight miles’ depth, has been forced upwards, and consequently, that the neutral zone is lower than that. But we know of no rocks at the surface except the truly eruptive, which have come from a greater depth than has granite. Had it come from the neutral zone, this would have made 2k equal from 7 to 84 miles, and therefore k would have 1 Nature, Vol. xx11. p. 392. GONG oOo SS (CO)