-94 PEOCEEDIFGS OE THE GEOLOGICAL SOCIETY. 



strata, or the atmospheric pressure was much greater. I may 

 remark also that where some of the earlier Archseans have been con- 

 siderably folded, and these plications are obviously anterior to 

 Cambrian times, the rocks appear to have bent more readily than 

 they afterwards did. This is what we might expect. In very 

 early ages of the earth's history, the rapid escape of heat would 

 cause frequent bending of the crust ; but the temperature would 

 increase quickly downwards from the surface, rendering the rocks, 

 as a whole, more plastic*. 



Pressure Metamorjphism. 



The aforesaid changes will be best understood after describing the 

 effects of pressure followed by more or less mineral change upon 

 diiferent kinds of rock. Let us take first the production of slaty 

 rocks from comparatively homogeneous shales. This subject has 

 been so admirably treated by my predecessor in this chair, Dr. 

 Sorby, that I need do little more than make a passing reference tc 

 it, and will merely state that my own investigations fully confirm 

 his statements concerning the presence, in many slates, of exceed- 

 ingly minute flakes of a micaceous mineral in large quantities. 

 These I think to be most probably, in the main, of secondary origin. 

 Perhaps the reason of their production may be that in these cases 

 the felspathic minerals, whose detritus was the principal constituent 

 of the slate, had not been so fully deprived of their alkalies as in those 

 where kaolin is chiefly present. Still, exogenous mica may be 

 often observed. The crystallization of the above-mentioned minute 

 mica-flakes is, no doubt, ceteris paribus, a consequence of the pressure 

 to which the rock has been exposed. They are most conspicuous and 

 best developed in regions which have obviously undergone great 

 folding and compressing, and in these they can sometimes be seen 

 to be most markedly developed where the pressure has been most 

 intense. Hence these glossy slates or phyllites may be regarded 

 as a first step towards the formation of a mica-schist. But it must 

 be remembered that it is a first step only, and between it and a 

 normal mica-schist there is a very wide interval, in which interme- 

 diate forms but rarely appear. Such forms also, when they do occur, 

 are found, not in any part of the series of well-marked crystalline 

 schists, but only (so far as T know) amoDg those to which from other 

 considerations we should assign a comparatively late date. Some 

 remarkable instances of these phyllites have been shown to me by 

 Professor Boyd Dawkins from certain localities in the Skiddaw Slate 

 of the Isle of Man. Prom specimens which I have seen I infer that 



^' From the above remarks, and others in this address, I may be supposed 

 to favour the idea of a community of structure in rocks of dissimilar origin. 

 I therefore think it well to say that my work has led me to the exactly 

 opposite conclusion, viz. that rocks of dissimilar origin have dissimilar struc- 

 tures. Of the alleged identities, some are mere superficial similarities, very 

 much of the " Monmouth and Macedon " type ; others are due to the structure 

 being too minute to be properly appreciated and distinguished ; others, again, 

 occur, as among the oldest rocks, where we are ignorant of their origin. I only 

 hold that an original structure can be subsequently obliterated. 



