140 , :^^QCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



horizontal. At their northern limit we find traces of considerable 

 dislocation, and we then cross a line of fault, and pass to the flank 

 of Moel Ddu, where the beds have the true N.N.E. strike of the 

 Carnarvon chain. The section I am about to notice commences at 

 the south end of the promontory called Gest, and ranging nearly 

 north and south passes through Tremadoc, and is prolonged to the 

 jank of Moel Hebog and Moel Ddu. It is along the south part 

 bf this line that Mr. Davis* found certain fossils which had escaped 

 my notice in 1831. I had the advantage of his assistance, during the 

 past summer, in examining the northern end of the section across 

 the line of fault above noticed. The entire section is as follows, and 

 is in an ascending order : — 



1. Commencing at the south end of the promontory, we first meet 

 with alternations of highly quartzose slates and gritty bands, in re- 

 peated alternations, and intersected by great veins of quartz, some 

 of which are metalliferous. Over these comes a great group of slates 

 with imperfect cleavage, and flags of a singular flaky structure, re^ 

 sembling the well-known pyritous flags of Festiniog. The series is 

 of great thickness, and is surmounted conformably by the greenstone 

 ridge of Moel y Gest. This series contains beds with innumerable 

 fucoids and many specimens of a large Lingula (first described by 

 Mr. Davis)f. There are many interesting mineral phsenoraena I 

 am compelled to pass over, especially near the junction of the slates 

 with the overlying greenstone. 



2. The great tabular mass I have called greenstone has many 

 modifications of structure I cannot notice. It appears to have been 

 erupted in a state of fusion, but no point of eruption is (so far as I 

 know) exposed to view. It is almost exactly parallel to the beds 

 above it and below it ; and having the same dip with the beds on 

 which it rests, it hangs at a considerable angle towards the north, and 

 so passes under the next group. 



3. Next follows a great group of dark earthy slates, occupying 

 the marshes above Tremadoc. Many parts of this group are with- 

 out fossils ; but in the upper portion of it we found Fucoids, Grap- 

 tolites, and a few Trilobitesj. It is mineralogically like the dark 

 earthy slates on the shores of the Menai Straits ; and, like these 

 slates, it contains large masses of pisoiitic iron ore (now extensively 

 worked), and beds of loadstone. Some of these masses are injured 

 by much-disseminated iron pyrites ; and such is the case with a 

 similar iron ore on the Carnarvon coast, north of the Rivals. In 

 the upper part of this group are trappean slates (^Schaalstein) and 

 other contemporaneous igneous products ; and the mineral phaeno- 

 mena well deserve a more detailed description. 



4. Next follows a Si-cond great terrace of igneous rock, almost 

 identical with that of Moel y Gest (No. 2). 



5. Then follows a series of slates, flags, and trappean shales. 



6. Then a third great band of greenstone, &c. 



* Journal of the Geological Society, vol. ii. p. 73. 



t Among the specimens ai-e Fucoids, Serpulites, Lingula, allin great abundance. 



X Fucoids, Graptolites foliaceus, G. Murchisonce, Asap}ms Powisii. 



