TRANSACTIONS OF SECTION C. 603 



it -would he almost impossible to describe in words, as they can only be learnt by 

 long-continued work and varied experience. I do not pretend to say that in the 

 case of a gi-it composed of fragments of about -02" diameter we can succeed in 

 identifying the parent rock of each individual, but I believe we can attain to a 

 reasonable certainty as to whether any large number of its constituents have been 

 furnished by granitoid rocks, by banded gneisses and schists, by fine-grained schists 

 or certain phyllites, by older grits or argillites, or by lavas and scoria. _ There 

 seem to be certain minute differences between the felspars from a granitoid rock 

 and from a porphyritic lava, and more markedly between the quartz grains from 

 the two rocks. The latter can generally be distinguished from the polysynthetic 

 grains furnished by certain schists or veins, and these not seldom one from another. 

 Obviously the larger the fragments the less, cateris paribus, the difficulty in their 

 identification. "When they exceed one-tenth of an inch the risk of important error 

 is, I believe, to a practised observer comparatively small. 



Obviously, also, the shape of the grains leads to certain inferences as to the dis- 

 tance which they have travelled from their original source, and as to the means of 

 transport, but into the details of this I must forbear to enter. I will merely remind 

 you that small angular' fragments of quartz are so slowly rounded, when trans- 

 ported by running water, that, if well-rounded grains appear in large numbers 

 in a sandstone, it seems reasonable to suppose that these are, in the main, wind- 

 drifted materials. 



Thus every rock in which the constituent particles admit of recognition and of 

 identification may be made to bear its part in the work of deciphering the past 

 history of the globe. Where the constituents have been derived from other rocks 

 we obtain some clue to the nature of the earth's crust at that epoch ; where the 

 locality whence a fragment was broken can be discovered, the nature, strength, and 

 direction of the agents of transport can be inferred. Some idea as to the structure 

 and surface contour of the earth in that district, and at that time, can be formed, 

 and thus the petrologist, by patient and cautious induction, may, in process of time, 

 build up from these scattered fragments the long- vanished features of the pre- 

 historic earth, with a certainty hardly less than that of the paleontologist, when 

 he bids the dry bones live, and repeoples land and sea with long-vanished races. 

 The latter study is in vigorous maturity, the former is still in its infancy ;_ so much 

 wider then is the field, so much more fascinating, to many minds, is the investiga- 

 tion. There are many districts which are without fruit for the palreontologist — 

 there are few indeed which, to the petrologist, do not offer some hope of reward. 

 The field of research is so wide that not one nor few men can gather all its fruits. 

 It needs many workers, and it is in the hope of enlisting more that I have ventured 

 to bring the subject before you to-day. 



Materials of the coarse?- fragmental rocks of Great Britain.^ 



I proceed now to give a brief epitome of the constitution, so far as I know it, 

 of our British grits, sandstones, breccias, and conglomerates. I shall exclude, as 

 involving too many collateral issues, the Post-Pliocene beds, and dwell more on 

 the earlier than on the later deposits, because the latter obviously may be derived 

 from the former by denudation, so that it becomes the more difficult to conjecture 

 the immediate source of the constituent particles. Further, in order to avoid con- 

 troversy on certain questions of classification, or for brevity, I shall occasionally 

 group together geological formations which I think separable. 



It may be convenient, however, to call your attention to the localities at which, 

 at the present day, granitoid rocks (many of which may be of igneous origin, but are 

 of very ancient date), gneisses, and crystalline schists are exposed in Great Britain, 

 as well as those where considerable masses of igneous rock, of age not later than 

 Mesozoic, occur. The former constitute a large part of the north-western and 

 central highlands of Scotland and of the islands off its west coast ; they are ex- 



• I have been obliged to exclude those of Ireland, as I have so little material from 

 that country, and for want of space have not dealt fully with those of Scotland. 



