HARDWICKE'S SCIENCE-GOSSIP. 



243 



the limestones in every other formation are always of 

 vital origin — that is, they have been formed by the 

 accumulation of coral sand and reefs, of shells, &c, 

 cemented, perhaps, by a still greater bulk of micro- 

 scopic organisms. The white chalk of Norfolk is 

 nearly as thick as one of these beds of Laurentian 

 limestones, and yet, to the naked eye, it offers no 

 explanation of its origin. It is not until you have 

 applied the microscope that you perceive it to be 

 almost entirely built up of the shields of animal- 

 cule, many of them of the same species as are now 

 living in the Atlantic ! If, therefore, the limestones 

 of every known geological period have been formed 

 by vital agency, one would imagine that those 

 limestones, whose organic remains had been oblite- 

 rated by the great heat to which they have been 

 subjected, might be reasonably put down to the 

 same origin. Again, the various phosphates, &c, 

 formed in these altered limestones, plainly tell of 

 animal life having been employed in elaborating 

 them. But, mighty though the transitions have 

 been through which the whole of the Laurentian 

 rocks have passed, all traces of fossils have not 

 been lost. The limestones yet contain myriads of 

 Eozoa, as plainly showing they were formed by its 

 agency, as a coral reef tells you how its bulk grew 

 to its present size. 



Twenty thousand feet of material had been 

 strewn along the bottoms of the Laurentian seas 

 in various places, the material varying according to 

 its neighbourhood to the mouths of rivers, &c, 

 whence it was brought. The solidification of 

 this mass took place contemporaneously with its 

 deposition. A great plutonic change then took 

 place, and what had been sea-bottom for ages, 

 existed as dry land. Then followed a period of 

 submergence, when it once more became sea- 

 bottom, aud had piled over it ten thousand feet of 

 extra material ! You ask how I know all this, and 

 I reply by pointing to you how the upper ten thou- 

 sand feet of rock lie tcnconformally to the lower 

 masses. By " unconformabiiity " I mean that the 

 dip of their beds is not the same, the lower being 

 different to the upper. This plainly shows that the 

 lower beds were uptilted before the upper were 

 formed, and that both series partook of the move- 

 ment which finally elevated the upper Laurentian 

 beds into dry land, in which state they remained 

 during the subsequent Cambrian epoch. 



You can readily understand how the Laurentian 

 rocks, being the first formed, must have undergone 

 more changes than any other, inasmuch as they 

 have had to partake of all that has gone on since 

 they originated. It is a wonder that we now find 

 any of them uncovered by rocks of subsequent 

 date, and we should not, had it not been for those 

 great atmospherical denudations'which have stripped 

 off miles in thickness of overlying rocks, so as to 

 expose those of an older date. The Laurentian 



strata have had, perhaps, miles in thickness of the 

 rocks of other formations piled above them. They 

 have had to undergo those great depressions which 

 eventually brought them so much under the in* 

 fluence of the earth's internal heat. Masses of 

 granite, trap, porphyry, &c., have been intruded 

 through them, and thus they have been squeezed 

 aud contorted in the most fantastic manner. The 

 sandstones, some of them five hundred feet in thick- 

 ness, have been so affected by heat as to become 

 quartz, or quartzite. Here, then, you have the 

 secret of my origin — the whole history of the 

 changes which brought about my present appear- 

 ance ! The limestones that were contemporaneous 

 with myself were altered so as to resemble loaf- 

 sugar, and had all, or nearly all, their organic 

 remains obliterated. The shales and slates became 

 transformed by heat, chemical change, and pressure, 

 into mica-schists, gneiss, felstones, &c. So that the 

 very peculiarity in dip, contortion, absence of 

 fossils, mineralogical changes, &c., which mark all 

 the rocks of [the Laurentian age, tell of their vast 

 antiquity; whilst the similarity in composition of 

 these rocks in all parts of the world, — in Ireland, 

 Scotland, and North America, as well as the preva- 

 lence of the same lowly-organized fossils in their 

 limestones, indicates they have passed through the 

 same transformations since they were contempo- 

 raneously deposited as limy muds, sands, and clays 

 along the floors of the primeval seas ! 



LUMINOSITY OE PLANTS, AND RETINAL 

 VARIABLE SENSIBILITY. 



" A GAIN, I always fiud, and should be glad to 

 -^- know if others do, that if the eye is fixed 

 upon a particular flower, the flashes are not seen, 

 while they are very visible the moment the eye is, 

 as it were, loosened, and allowed to wander over 

 the flowers." Such is the Note-and-Query of 

 " E. T. S." in the August number of Science- 

 Gossip, speaking of a phenomenon "seen in the 

 dusk when the fading light is somewhat confusing." 

 Whilst reading the above. I was at once reminded 

 of a fact narrated by the late Sir John Herschel, 

 in connection with the subject of sidereal astronom v . 

 He states, " There is a group of stars called the 

 Pleiades, in which six or seven may be noticed, if 

 the eye be directed full upon it, and many more if 

 the eye be turned carelessly aside while the attention 

 is kept directed upon the group." 



I think there is an analogy between the botanical 

 and the astronomical occurrences. The explanation 

 given by the illustrious astronomer is this : that 

 " the centre of the visual area is by far less sen- 

 sible to feeble impressions of light than the exterior 

 portions of the retina. Pew persons are aware of 

 the extent to which this comparative insensibility 



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