ADDRESS. 15 



Bat all metamorphic rocks are not of the same character with the 

 gneisses of the Lower Laurentian. Even in the Middle and Upper Lan- 

 rentian we have metamorphic rocks, e.g., quartzite and limestone, which 

 must originally have been ordinary aqueous deposits. Still more in 

 the succeeding Huronian and its associated series of beds, and in the 

 Lower Palfeozoic, local metamorphic change has been undergone by rocks 

 quite similar to those which in their unaltered state constitute regular 

 sedimentary deposits. In the case of these later rocks it is to be borne 

 in mind that, while some may have been of volcanic origin, others may 

 have been sediments rich in undecomposed fragments of silicates. It is 

 a mistake to suppose that the ordinary decay of stratified siliceous rocks 

 is a process of kaolinisation so perfect as to eliminate all alkaline matters. 

 On the contrary, the fact, which Judd has recently well illustrated in the 

 case of the mud of the Nile, applies to a great number of similar deposits 

 in all parts of the world, and shows that the finest sediments have not 

 always been so completely lixiviated as to be destitute of the basic matters 

 necessary for their conversion into gneiss, mica-schist, and similar rocks 

 when the necessary agencies of metamorphism are applied to them 

 and this quite independently of any extraneous matters introduced into 

 them by water or otherwise. Still it must be steadily kept in view that 

 many of the old pre-Cambrian crystalline rocks must have been different 

 originally from those succeeding them, and that consequently these last 

 even when metamorphosed present different characters. 



I may remark here that, though a palaeontologist rather than a 

 lithologist, it gives me great pleasure to find so much attention now given 

 in this country to the old crystalline rocks, and to their study micro- 

 scopically and chemically as well as in the field, a work in which Sorby 

 and Allport were pioneers. Aa a pupil of the late Professor Jameson, 

 of Edinburgh, my own attention was early attracted to the study of 

 minerals and rocks as the stable foundations of geological science • and 

 as far back as 1841 I had learnt of the late Mr. Sanderson, of Edinburo-h 

 who worked at Nicol's sections,' how to slice rocks and fossils ; and since 

 that time I have been in the habit of examining everything with the 

 microscope. The modern developments in this direction are therefore 

 very gratifying to me, even though, as is natural, they may sometimes 

 appear to be pushed too far or their value over-estimated. 



That these old gneisses were deposited not only in what is now the 

 bed of the Atlantic, but also on the great continental areas of America 

 and Europe, anyone who considers the wide extent of these rocks repre- 

 sented on the map recently published by Professor Hull can readily under- 

 stand.2 It is true that Hull supposes that the basin of the Atlantic 

 itself may have been land at this time, but there is no evidence of this 

 more especially as the material of the gneiss could not have been detritus 

 derived from sub-aerial decay of rock. 



' And I believe at Witham's also. = Tratu. Royal Irish Academy. 



