222 



HA RD IVICKE'S SCIENCE-G 0SS1P. 



of the ocean's floor, for a large series of beds may be 

 traced from this point to the southern side of the 

 island, all of later birth than the starting-point. 



Another bed in ascending order was doubtless 

 originally composed of much the same class of 

 organisms as the preceding ; but careful inspection 

 discloses a fact which is not without value. All the 

 more delicate and fragile forms have disappeared, but 

 here and there a more than usually robust one still 

 retains its outline. The rock assumes a more dis- 

 tinctly crystalline texture. What is the meaning of 

 this ? Here is a minute fissure in the section ; 

 examine closely, and you look down upon a miniature 

 river of calcite carrying with it microscopical grains 

 of foreign material ; here at a point it has met with 

 some obstruction : the stream bifurcates and the 

 branches given off are again sub-divided until they 

 become quite indistinct. This tiny flood is doubtless 

 the product of the dissolution of the organisms 

 originally contained, and the practical result is the 

 re-deposit and crystallisation in minute granules 

 similar to the saccharoidal marbles. 



Rutley says, " It seems impossible, in many cases, 

 to say whether this structure in limestones has been 

 due to metamorphism engendered by contact or 

 proximity of eruptive rocks, or whether it is owing to 

 other causes." * Here, then, is a certain amount of 

 evidence that it has been possible to produce this 

 change by the simple agency of water without calling 

 in the assistance of the igneous element. Certainly 

 ho eruptive rocks occur within reasonable distance of 

 this locality, and even had their influence operated, 

 it would be still more difficult to account for their 

 action on this particular bed, while other older, but 

 not far removed, beds in vertical depth, escaped. It 

 is evident a continuance of these conditions would 

 have removed all trace of life from this bed, and 

 shows it is unwise to doubt the organic origin of a 

 bed, although there is no distinct proof obtainable. 



Lycll, in his " Student's Elements of Geology," 

 says, "In the upper part of the mountain limestone 

 group in the south-west of England, near Bristol, 

 limestones having a distinct oolitic structure alternate 

 with shales." South Wales also furnishes evidence 

 of certain beds having this structure ; nor is it of local 

 occurrence, but appears to have prevailed over a very 

 large area. Both the holmes have beds which are 

 distinctly oolitic to the unaided eye ; but the micro- 

 scope reveals the fact, the grains are not surrounded 

 with the number of lamina: that specimens taken from 

 other districts exhibit. The fragments are of larger 

 size,, have a rolled appearance, and the external 

 deposit appears less pure than in other examples to 

 be named. 



At Rhiwbina, on the south crop, excellent examples 

 of this structure occur. The enclosed fragment may 

 consist of a minute foraminifer, a fragment of polyzoa, 



* " Study of Rocks," by F. Rutley, F.G.S., page 2S7. London: 

 Longmans, Green and Co. 1879. 



or portion of an ossicle of crinoid. All are sur- 

 rounded by the concentric lamina; of minute crystals 

 of calcite ; the individual grains, however, are much 

 smaller than those taken from the holmes. 



The most perfect example of this structure, how- 

 ever, is met with in the north-east of the district in 

 the neighbourhood of Gilwern. Here the nucleus 

 appears as a mere speck, and as many as fifteen 

 distinct lamina: surround it, the whole grain measuring 

 about the one-fiftieth part of an inch. The interspaces 

 are filled with crystals of much larger growth ; the 

 whole bed has almost a perfectly white appearance, 

 and would furnish a building material of great beauty 

 and durability. 



Who shall venture to say what were the conditions 

 which caused these differences to exist side by side? 

 Were these granules quickly crystallised, their growth 

 being arrested as they approached each other? or 

 were they subject to disturbance and consequent 

 movement among themselves, and thus the growth 

 of encrusting crystals was interfered with, while the 

 interspaces, being the last to consolidate, were suf- 

 ficiently quiet to permit the crystals to assume much 

 larger proportions ? 



One peculiarity worth recording as occurring at 

 Rhiwbina should be stated. There is at this point a 

 deposit of ironstone made up entirely of the altered 

 ossicles of encrinites, polyzoa, and allied forms ; it 

 has the deep rich colour of haematite ore, and some 

 years since was worked to a limited extent, but for 

 some cause has been abandoned. It occurs at or near 

 the base of the limestone, and in its way is most 

 interesting. 



Few rocks present more points of interest than the 

 limestones, or are of greater value to mankind. If 

 we view them as the preservers and repositories of 

 the myriad forms of ancient life, they cannot fail to 

 impress the mind with the idea of the immensity of 

 lime required for the accumulation of the vastly thick 

 deposits of different ages. Their distribution in time 

 and space is also interesting, as indicating a continuity 

 of conditions through countless ages. In arts, manu- 

 factures, architecture, and agriculture, their uses are 

 too well known to require recapitulation here, and 

 the object of this paper will have been attained by 

 placing on record one or two features of interest 

 occurring at points in this locality not often visited by 

 knights of the hammer. 



Cardiff. 



Sycamore Cotyledons. — Referring to Mr. Jeffery's 

 query in last month's SciENXE-GossiP (p. 215), 

 regarding the colour of sycamore cotyledons, I 

 beg to state that I gathered a number of famarus 

 in the autumn, two years ago, and after keeping 

 them all winter in the husks, found, on opening them, 

 that the cotyledons were as green as when first taken 

 from the trees. — J. Ballantyne. 



