HARD WICKES S CIENCE- G OS SIR 



J 33 



and some aqueous deposits. The great difficulty in 

 this district lies in the fact that the rocks are isolated 

 and are not near any Cambrian rocks, but are in 

 contact with Triassic or Carboniferous rocks on all 

 sides. 



7. Corntcall. — The new Eddystone lighthouse is 

 built on a gneiss of a type which can only be matched 

 among pre-Cambrian rocks. There seem to be in 

 South Cornwall curious hornblende schists and other 

 rocks which may hereafter be referred to this period. 



These two or three great systems of pre-Cambrian 



was intensely heated, and when the seething of the 

 primordial ocean might have given rise to the 

 crumpling and contortion visible in these rocks. ' 



The strictly uniformitarian school believed these 

 rocks were laid down as ordinary sediments, and 

 were subsequently metamorphosed by heat, water, 

 and pressure, — agencies now in action. 



Some geologists are, however, now going back to- 

 the old ideas, and Dr. S terry Hunt has gone so far 

 as to state that "all gneisses, hornblende and 

 micaceous schists, &c, are of Neptunean origin, and 



Fig. 82 —Typical perlites in Meissen pitchstone. Showing passage 

 of microliths through perlites. 





Fig. 83.— Pebidian felsite (Wrockwardine), showing perlites 

 (Allport), X 10. 



Fig. S4. — Perlites of Meissen, showing their 

 dependence on joints (Allport), X 6. 



.3 ff/^—c 



Fig. 85.— Kremnitz perlite, with crystals (c), and 

 spherulites (s). The microliths pass through the 

 latter, but not the former (Allport), X 20. 



Fi g. 86. — Pre-Cambrian felsite (Wrockwardine), 

 with bands of spherulites, sometimes round 

 crystals, and traversed by microlithic streams 

 (Allport), X 2. 



rocks contain a great thickness of strata, and so must 

 represent a long lapse of time; yet, when we ask 

 for the life history of the period, they are all silent, 

 with the doubtful exception of the Wrekin quartzite, 

 possibly pre-Cambrian in age, which has yielded to 

 Dr. Callaway an obscure trace of a worm burrow. 



A word on the various opinions which have been 

 expressed as to the origin of these old rocks may not 

 be out of place. 



The earliest school of geological thought referred 

 all the crystalline schists to the time when the earth 



are not primarily due to metamorphosis of ordinary 

 sediments." "The chemical and mechanical con- 

 ditions under which these rocks were deposited and 

 crystallised . . . have not been reproduced to any 

 great extent since Palaeozoic times." 



We have yet however so very much to learn about 

 the mere succession, position, and character of these 

 rocks, that it is, perhaps, mere waste of time to 

 speculate as to their mode of formation, or to draw 

 conclusions from them as to the state of the earth in. 

 those very ancient times. 



