556 
NA DORE 
| March 31, 1870 
Prismatic Ice—Sandstone Boulder in Granite 
WALKING over the Rough Tors of Dartmoor a few days 
since in company with a friend, Mr. Walter Morrison, M.P., 
we observed two phenomena which it appears to us should be re- 
corded, since if they have been met with before, we believe 
thac the occurrence of either is infrequent. 
In asmall roadside quarry, most probably excavated for the 
purpose of obtaining stones for metalling the road, we observed 
some granite undergoing the process of decomposition. The 
stratification of the superincumbent mass struck us as being 
peculiar, and we entered the quarry to examine it. 
The near examination exhibited to us that the surface of the 
granite had been elevated to the extent of an inch and a half 
from the rock by means of ice. The whole of the ice, which 
formed a very extensive bed, was made up of a series of small 
needle-like prisms, which stood upon their end like minute basaltic 
columns. ‘These prisms were all perpendicular to the surface of 
the rock on which they stood ; that is, those that were situated 
on the side of the quarry projected horizontally, while those 
at our feet stood in a position vertical to the floor. These 
latter, instead of being on a rock, were upon the surface of 
granitic gravel, the debris of the surrounding rocks. And 
in the same manner as the ice mass had forced out the sur- 
face of rock at the sides of the quarry, it had raised up the entire 
surface of the gravel; and this so completely, that we only 
observed it in consequence of examining it for comparison, since 
the ice was strong enough to bear our weight when not in- 
creased by irregular motion. 
It was to us new and interesting to find the ice assuming 
this basaltic form, and occupying the position in which 
we found it, in relation to the rock and gravel over which 
a. Débris on the surface of the 4 Crystalline ice. 
c. Surface of rock, 
it lay. Water in freezing will put on almost any description 
of crystalline appearance; there is no reason then why up- 
right needle-like prisms may not be as frequent as any other, 
but that it should put on this form in this position can only 
be explained when we know how the water that formed the 
ice got to that position. In trying to obtain this information, 
we observed that the lines of the stratification in the softer parts 
corresponded above and below the ice, and that those on the 
surface of the ice agreed with those at the base. This would 
demonstrate that the water did not arrive in that position as 
water flows over the surface of the rock, Had that been the 
case, the outer crust would, if it had not fallen off, have been 
moved a degree lower, which would have coincided with the spot 
at the base from which it was removed. The only way in which, 
therefore, the formation of the ice appears possible, consistently 
with existing facts, is by the water having percolated through the 
decomposing granite rock. There, oozing gradually, it only froze 
when it came to the surface, while the weight of water behind had 
power to lift off the external crust of rock. This again was 
forced outward by the water still pressing behind, the whole 
freezing only when it passed out through the rock. One and, I 
must admit, a strong objection to this idea lies in the circum- 
stance that the gravel at the bottom of the quarry was elevated in 
the same manner ; there cannot have been at the bottom of the 
quarry a weight of water from behind pressing the basaltic prisms 
outward. Whatever may be the cause of the phenomenon, there 
can be no doubt but that it must be an immensely powerful 
agent in the disintegration of rocks.* 
The second feature that appears to be worthy of being recorded 
is that of a water-worn sandstone pebble imbedded in a mass of 
granite. On the summit of the hill near Warren Tor, we came 
upon the remains of a cairn, most of the stones of which had been 
carted away, apparently for the purpose of being built into a 
house that has recently been erected near it. The stones 
generally consisted, like those common to all the cairns of the 
moor, of angular pieces of granite of irregular shape and size, but 
mostly of such proportions as could be conveniently conveyed by 
hand. Lying near what appeared to haye been the centre of the 
cairn were two other slabs of granite of much larger proportions. 
These were about four feet long by two and a half broad, and 
from eight to twelve inches thick, and appeared to us to have been 
the remains of the kistvaen, over which the stones were heaped. 
In one of these granite slabs my companion pointed out to me 
the presence of a large water-worn boulder pebble of sandstone. 
The pebble was about ten inches long by five wide at its longest 
and broadest diameters. It projected from the surface of the granite 
some two inches, or thereabout, but the form of the stone was 
suggestive of the larger part being still imbedded in the granite. 
The stone narrows somewhat towards one extremity, and at that 
extremity it is cracked across. The pebble is of blue sandstone, 
smooth but not polished on the surface, and at its contact with 
the granite exhibits no alteration of character. 
The granite is of the grey colour that is so common on Dart- 
moor. It is of a coarse quality, containing numerous large 
crystals of felspar, locally known by the name of horses’ teeth. 
The granite generally appears as if it had been deposited round 
the pebble, the crystalline character taking a bend somewhat 
conformable to the curves of the pebble. 
The original rock from which the pebble was derived it is 
difficult to determine. In its general appearance the pebble 
bears a resemblance to those found on the pebbleridge of Northam 
Burrows, in North Devon. Unfortunately the granite slab itself 
is not in its natural bed. But since it forms part of an ancient 
monumental erection, and was placed in its present position long 
before the period when the transition of heavy weights could be 
easily accomplished, it is more than probable that the slab was 
procured from the neighbouring Tor. 
The surface of the granite slab is very clean, being free from 
lichen and dendrital growths. But that it was weather-worn 
before it was placed in its present position, I think that there can 
be little doubt. Had it been artificially split, supposing that our 
ancient people had the knowledge and skill to have split the 
rock, they could not have done so without breaking the pebble 
as well. 
Mr. Sorby demonstrated some five or six years since the pre- 
sence of water in the hollows existing in crystals in granite, and 
thus came to the conclusion that granite must have been formed 
under a heat of less than 500 degrees. This heat is still, it ap- 
pears to me, sufficient to convert all the water upon the surface 
of the earth into steam. But the rounded and smoothed cha- 
racter of the pebble of which we are speaking is evidence that 
water was present in its liquid state when it was rubbed into its 
present rounded form. It also demonstrates that the pebble was 
in existence as a hard rock before the granite was formed. Thus 
it would seem clear that when this granite was formed, the tem- 
perature of the earth must have cooled down to below the boiling 
point of water. 
Plymouth, March. C. SPENCE BATE 
Apparent Size of the Moon 
THE public have what are called “ views” on everything in the 
world and out of it. De Morgan well says that, on the question 
‘“whether there be volcanoes on the unseen side of the moon 
larger than those on our side,” the odds are, that the first 
comer has a pretty stiff opinion in three seconds. Does anyone 
believe that the moon is made of green cheese? One woman 
hailing from this enlightened neighbourhood told my wife that 
she thought the moon was made of blood! She was not a Latter- 
day Saint, either. 
If opinions differ as to the moon’s constituents, they are not 
very divergent as to her apparent size. Mr. Thomas K. Abbott, 
of Trinity College, Dublin, in his singularly learned and able 
* Since this was written I have seen Mr. Bonney’s letter on Prismatic 
Ice. The wind during our walk was N.E. The quarry lay ina sheltered 
position. The temperature was probably as Mr. Bonney states. 
woe 
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