HA R D \V1 CKE 'S S CIENCE -GOSS IP. 



173 



tlie area, by tlie marine muddy sediments now 

 hardened into tiie slates or "killas," so largely 

 developed over South Devon 'and Cornwall. Vol- 

 canic agency was rife then, and some ash-showers 

 and lava-flows are interbedded with the slates. Suc- 

 ceeding this period the eruptive force was subdued, 

 manifesting itself only in the slow subsidences of the 

 area. Extensive growths of coral now took place, 

 as represented by the limestone. Again, the waters 

 became muddy, the conditions altered, and we pass 

 into the Culm-measure shales, sandstones, and grits, 

 with here and there beds of anthracite, or culm, and 

 occasional band> of limestone. 



Succeeding this period, great volcanic activity was 

 manifested. The granitic bosses, of \\hich Dartmoor 

 is one, were, intruded amongst the rocks, both 

 Devonian and Cuhn-measures, which have since 

 been extensiuely denuded from above them, while 

 Elvan dykes and other veins of igneous matter were 

 thrust out here and there amid the slates and 

 limestones. 



Portions of some large lake then occupied the area 

 in which the Triassic rocks, so Prof. Ramsay tells 

 us, were deposited. The area may ha\e been con- 

 tinuously upheaved in this tract, which was certainly 

 not the case in others. It is, however, scarcely 

 probable that any of the Liassic or Oolitic sediments 

 were spread over the area. 



Not until we come to the Cretaceous period do we 

 again find evidence of extensive deposition or evidence 

 of submergence. ' Then the sandy sediments of the 

 Greensand were formed along the eastern margin of 

 Dartmoor; liut how far they extended to the south 

 and to the south-west is uncertain. The chalk 

 must, it is considered, have spread over the whole of 

 Devon, for it required a deep sea for its formation. 

 It is quite possible, however, that Dartmoor remained 

 as an islet above water, and this would have yielded 

 the pea-like grains of quartz which are found in the 

 lowest beds of chalk in Devonshire and Dorsetshire. 



Enormous denudation must have taken place since 

 this period, in Tertiary times, of which the gravels 

 and superficial soils are but feeble relics. The 

 Miocene deposit of Bovey Tracey tends to show that 

 much must have been denuded in Eocene times ; but 

 this period, small as it may seem in comparison with 

 other geological epochs, must itself have been of 

 great duration. 



In more recent times — Pliocene, Glacial, and Post- 

 Glacial — the area can scarcely have remained \\\\- 

 affected by the changes, of which elsewhere in the 

 British isles, we have such conspicuous records. 

 Forty years ago, Mr. Godwin-Austen hinted that the 

 meagre list of shells from the raised beaches pointed 

 to the period having been "one less favoui-able to 

 the development of marine life, owing, perhaps, to 

 a lower temperature." And he added that the 

 broken-up or detrital edges of the slate rocks, a 

 feature frequently to be observed, might have been 



produced by agencies in a period having a lower 

 temperature and attended by the action of deeper 

 searching cold."' 



But the connected history of these later deposits, of 

 raised beaches, submarine forests, and caverns, 

 recording as they do many of the ups and downs of 

 modern geological change, remains yet to be told. 



The scenery itself is the general result of the 

 changes that have affected the area throughout all the 

 geological periods. The consolidation of the strata 

 after deposition, their induration and elevation, their 

 disturbance and dislocation, produced the ground- 

 M'ork upon which at various times the agents of 

 destruction have operated. 



The features of the coast-line, and the features 

 inland, are the results of marine and subaerial de- 

 nudation acting on rocks of unequal hardness, and 

 the direction of which forces has been modified more 

 or less by the disturbances which have affected the 

 rocks. Bays and promontories are formed, like hill 

 and dale, by the alternatron of hard and soft rocks, 

 the latter having been niore easily worn away than 

 the former. The granite of Dartmoor has Iseen up- 

 heaved to its present elevation, Init the tors and 

 other fantastic forms winch it assumes are the results 

 of subaerial denudation. The igneous dykes which 

 often form little conical hills, owe their present 

 features to the fact that they are better capable of 

 withstanding denudation than the surrounding slates, 

 and none, not even Brent Tor, so Mr. Rutley 

 informs me, have any immediate relation in outline 

 to the old volcanic features of which they are the 

 relics. Were we, however, to enter into any further 

 discussion of this subject, to attempt to trace out the 

 origin of the valleys of the Tamar, or of our " English 

 Rhine," the Dart, we should have to dwell upon 

 more of the local details of structure than the space 

 allotted to this sketch would permit. 



BOTANICAL NOTES IN THE 

 NEIGHBOURHOOD OF CADER IDRIS. 



FEW districts present so charming a diversity of 

 rock, wood, and water, of grand mural preci- 

 pices and craggy heights, of rich undulating woods, 

 of dark solemn lakes, leaping streams, and far- 

 reaching estuary, as the picturesque country about 

 the quaint Welsh town of Dolgelly, anciently written 

 Dolgellau. Weeks might be spent in exploring the 

 fine streams that come tumbling down between the 

 mountains, in ascending the many rocky heights, or 

 in reaching the shores of the lakes ; some beautiful 

 exceedingly, some grandly rocky, others the picture 

 of calm but stern solitude. 



In June of 1876 I spent four days at Dolgelly with 

 a scientific friend, and subsequently re-visited that 



* Trans. Gcol. Si'C, 2nd Ser. , vol. viii. pp. 437, 442; see 

 al'^o Mackintosh, Qj(art. yoiirn. Geol. Soc, vol. xxiii. p. 326. 



