Correspondence — Dr. C. Callaway — B,ev. 0. Fisher. 237 

 C O I^I?-:E S IPOZsTHDIE i5ro s . 



THE PEE-CAMBEIAN EOCKS OF ST. DAVIDS AND OF BOHEMIA. 



Sir, — I cannot find tliat the letter of Dr. Hicks in your last issue 

 materially strengthens the proof for the Dimetian age of the 

 gneissic series which underlies the Bohemian Pebidian. Dr. Hicks 

 maintains that the St. Davids Dimetian is a true gneiss. I cannot 

 of course say that the rock in the few sections which I did not see 

 is not foliated ; but I saw no true foliation in the principal localities 

 named in his papers, and I cannot discover anything about foliation 

 in the microscopic descriptions of Mr. T. Davies, Prof. Bonuey, and 

 Mr. Tawney. It is at any rate certain that if these rocks are schists, 

 their foliated structure is of the obscurest possible character, and 

 quite unlike that of the true gneisses. 



I quite agree with Dr. Hicks that we are not to expect " absolute 

 identity," but I deny that there is even a " general resemblance " 

 between the Dimetian of St. Davids and the gneissic rocks of 

 Bohemia, so far as we can judge from Mr. Marr's descriptions. Nor 

 do I think that a similarity of the conditions of deposit, even if 

 proved, goes for much. I presume that most arenaceous rocks, from 

 the Tertiary downwards, were laid down in comparatively shallow 

 water. 



That the Bohemian gneiss unconformably underlies the Pebidian, 

 simply proves that it is pre-Pebidian. I do not deny its Dimetian 

 age : indeed, I think it highly probably that Mr. Marr is right ; but, 

 as we do not yet know how many gneissic series lie below the 

 Cambrian, I demur to the assumption that any Archsean gneiss group 

 which is not Lewisian must be Dimetian. Any resemblance to the 

 newer gneiss of the Highlands can have no decisive value in our 

 present uncertainty of the age of that formation. These Archaean 

 groups are a very complicated study, and more haste may sometimes 

 prove to be the worse speed. The researches of Dr. Hicks have 

 done much towards unravelling the Archgean mystery, but we must 

 work along our clues with great caution, else we shall become the 

 sport of the Philistines who would condemn us to grind in the 

 prison-house of an eternal Siluria. C. Callaway. 



"Wellington, Salop, March btli, 1881. 



OBLIQUE AND OETHOGONAL SECTIONS. 



Sir, — In my short notice about the section of a folded plane there 

 is an error which Mr. Day has not pointed out. I did not expect 

 that what I had written would have attracted attention ; but since it 

 has done so, 1 may ask to be allowed to sa}^ in the sixth line from 

 the bottom of p. 21, dele " =^," and in the fifth, for "^" read 

 "EAB=d suppose." 



I cannot exactly see that Mr. Day's proof gives my second equation, 

 because his a, j3, and ^ do not appear to be the same angles as iu 

 my demonstration. 



The method of the shadow is ingenious and of course correct. 

 Haklton, 4<A March, 1881. 0. FlSHER. 



