1852.] SEDGWICK ON THE LOWER PALAEOZOIC ROCKS. 



147 



felstone-porphyry, and partly buried under drift. The Harlech grits 

 make a well-defined, and sometimes a very striking- feature in the 

 chains of Caernarvonshire and Merionethshire. They may be traced 

 almost continuously from Aber, and thence on the eastern side of the 

 line of slate-quarries into the great precipices of Craig Goch ; forming 

 great gnarled masses of rock as coarse as millstone-grit, often beauti- 

 fully jointed, sometimes with traces of cleavage-planes, and often 

 alternating with very thin bands of chloritic slate. Again, they are 

 well seen near Trowsfynydd, and afterwards on both sides of the great 

 Merioneth anticHnal ; and they form the most striking features of 

 the Rhinog Fawr chain, dipping towards the N.W. But this dip is 

 reversed by a synclinal curve, and the same great beds of grit are 

 brought to the coast at Barmouth and Harlech, bearing within their 

 trough some of the lower beds of the Festiniog group. On this latter 

 point I ought not, however, to write with confidence, as I have never 



Tabular View of the Falceozoic System. 



I{ 



Permian series. 

 ;3 L Carboniferous series. 



f Petherwin group.. 



Q •{ Devonian series 



Silurian series 



1^ 



[_ Cambrian series 



Metamorphic. 

 Granite. 



Caithness group ... 

 Plymouth gi'oup ... 



6. Ludlow group.., 



5. Wenlock 



group . 





"4. Caradoc 

 group.. 



3. Bala group. 



2. Festiniog 

 group.... 



1. Bangor 

 group. 



{Petherwin slate and Clymenia 

 limestone. 

 Marwood sandstone. 

 {Hereford sandstone, marl, and 

 cornstone. 

 Dipterus flags, 

 r Dartmouth slate. 

 ■I Plymouth limestone and red 

 [^ grit, and Liskeard slate. 



ft.* 



{c. Upper Ludlow 400 

 5. Ayraestry limestone ... 100 

 a. Lower Ludlow 500 



r c. Wenlock limestone ... 100 



I i. Wenlock shale 800 



1 a. Lower Wenlock, or 

 [ Woolhope limestone 100 

 r a. Caradoc sandstone, 

 \ limestone, and shale.. 1500 



b. Upper Bala (including 



Bala andHirnant lime- 

 stones, shale, flagstone, 

 and conglomerate) ... 4000 

 a. Lower Bala; dark slates, 



flags, and grits 4000 



c. Arenig slates and por- 



phyry 7000 



5. Tremadoc slates 1000 



a. Lingula flags 500 



i. Harlech grits 500 



a. Llanberris slates 1000 



* These measurements, like those in the preceding table of the Cumbrian 

 Rocks, p. 141, are merely approximative. 



l2 



