314 T. Mellard Reade— Glacial Geology. 



on a more distinctively marine character, until at Nevin they assume 

 the ordinary form of stratified drift-sands with shell fragments. 



Significance of tlie polished quartz grains. 



It will be necessary to turn aside here to point, out the value 

 I attach to the discover}' of the prevalence of rounded quartz sand 

 in the Till. I have been making a very exhaustive examination of 

 sands from various parts of the world, and I find that contrary to 

 received ideas marine sands are, on the whole, much more rounded 

 than river sands. Indeed, river sands are generally little rounded, 

 and, excepting in a river like the Amazon, I can find no parallel to 

 the rounding of the grains of sand now dredged from the Irish Sea. 

 Blown-sand of sand-dunes is not distinguishably more worn than 

 the sand of the shore from which it is derived. When we find 

 these highly-polished grains mixed up with angular and little worn 

 fragments of Snowdonian rocks forming the bulk of the Till of 

 the coastal plain, it is evident that they are of sea origin, and serve 

 as certainly as the fragmentary shells of Molluscs to declare this 

 fact. I consider this an important discovery, which may, taking 

 surrounding conditions into consideration, help to settle many dis- 

 putes as to the marine or freshwater origin of certain deposits. lu 

 the present case, of course, the rounded grains can tell us no more 

 of how they got to the high-levels than the shells of the Molluscs, 

 but more of this presently. 



Distinction between the Marine Drift of the Plains and Mountain Drift. 



Where the Marine Drift of the plains approaches a mountainous 

 district, and the base can be seen, it usually lies upon a Till made 

 up almost wholly of local materials derived from the mountains. 

 In some cases the Marine Drift is absent. At vai'ious points along 

 the coast of Merionethshire, and in Mid-Wales, the local Till is 

 prominently displayed in coast sections, but where a large estuary 

 valley opens out on to the coast, and sections display the nature of 

 the Drift, we find, as in the neighbourhood of Towyn, distinctly 

 Marine Drift in some places, and a mixture of Mountain and Marine 

 Drift in others. The stony Till clings to the mountain sides, the 

 Marine Drift comes in more strongly as it is distant from the 

 mountains. This is a rule to which, personally, I have seen no 

 exceptions. 



Theories of Origin. 



Most of the details upon which the preceding sketch is founded, 

 with the exception of those relating to the North and Mid-Wales 

 coast, have been published. It is very necessary that details should 

 be published, but the Drift is so complicated that it is very difficult, 

 if not impossible, to describe it geologically in the sense that other 

 formations, both older and younger, can be described. It is this 

 that has beset the path of most glacialists, and the attempts to 

 reduce the Drift deposits to geological order and system have been 

 the fruitful source of many failures. 



