542 Belt— On the " Lingula Flags." 



tlie junction with the lower blue beds some bands of blue slate 

 occur, interstratified with the black, but higher up they are entirely 

 black. With the exception of a thin layer of black slate in the 

 Festiniog Group, and which has been noticed but at one spot, the 

 Upper Dolgelly beds are the only black slates in the district, although 

 the term has been applied to the dark blue rocks of the Menevian 

 and Maentwrog Groups. These, however, are never black, and 

 when scratched show a white streak. The Upper Dolgelly beds are 

 not only black, but their streak also is black. A careful examination 

 of all the rocks of the Dolgelly district enables me to state that, 

 with the trivial exception mentioned above, there are no other beds 

 with a black streak. It is only since I established this fact that I 

 have been able to map out the beds in the highly disturbed district 

 to the east of Ehobell-fawr, and on Mynydd Gader. Geologists 

 who have attempted to unravel the intricacies of such a disturbed 

 country as that around Dolgelly, where the strata are faulted and 

 contorted, altered by intrusive igneous rocks, and often so shattered 

 and cleaved that it is useless to search for fossils, will appreciate the 

 value of the discovery of a test which enables us to identify a well- 

 defined set of strata however it may be fractured and cleaved. 



The Upper Dolgelly beds are characterized by a great many 

 Trilobites, none of which are found in the strata above or below. The 

 species found in the Dolgelly district are ConocorypJie (solenpleura ?) 

 abdita, Sal., C. Williamsonii, sp. n., C. longispina, sp. n., Peltura 

 scarahceoides, Wahl., SphcerophtJialmus hisulcaius, Vh.il., S. Jiumilis, Phil., 

 Agnostus princeps, Sal., A. trisectus, Sal., and A. obtiisiis, sp. n. Besides 

 the trilobites, a few shells belonging to the genera Orthis, Lingulella, 

 and Obolella, are found. The Orthis is 0. lenticularis, Dalm., ac- 

 cording to Salter; but the others have not been described. The 

 Lingulella is, however, very like L. Davisii, McCoy, from the Festiniog 

 beds. 



Near Tremadoc the following additional Trilobites have been found 

 in strata at or about the same horizon as the above : — ConocorypJie 

 invita, Sal., Dikelocephalus (?) celticus, Sal., and D. (?) discoidalis, Sal. 

 The Upper Dolgelly beds are about three hundred feet thick. 



The Dolgelly Group, comprising the Upper and Lower Dolgelly 

 beds, is altogether about six hundred feet thick. A very fine section 

 of the beds is exposed along a brook falling into the Mawddach at 

 Ehiw-felyn. From thence, with many dislocations, they curve round 

 the eastern side of Ehobell-fawr to Blaenau. Half a mile south-west 

 from Dolgelly the black beds are well developed up the ravine through 

 the grounds of Bryn-y-gwin to Bran-y-gader. where they are over- 

 laid by Lower Tremador beds, with Didyonema fenestrata, Sal. On 

 Mynydd Gader they overlie the Upper Festiniog beds, and are 

 followed conformably by Tremadoc strata containing Asaphus in- 

 notatus, Sal., Niohe Eomfrayi, Sal., Conocoryphe depressa, Sal., and two 

 species of Protospongia. 



The Lower beds are not known to exist except in the Dolgelly 

 district. They will, however, probably be discovered around Tre- 

 madoc, as in the Jermyn Street Museum there is a tail of Parabolina 



