162 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



to a mile, vertically and horizontally. Section No. 1 passes from 

 Mochras Island in Cardigan Bay, over Cader Idris, Radnor Forest, 

 and Hanter Hill, to the Old Red Sandstone near Fern Hall, south of 

 Kington, Herefordshire. It is about 65 miles in length. 



No. 2 commencing at Llanfair-is-gaer, Menai Straits, passes over 

 Snowdon, Moel-wyn, Gors-goch (near Trawsfynydd), Aran Mowddwy, 

 and Newtown, Montgomeryshire, and the Upper Silurian rocks and 

 Old Red Sandstone of Clun Forest, Wigmore Valley, &c., near 

 Ludlow. It is 90 miles in length. 



No. 3 passes across the Shelve and Longmynd country to the 

 Brown Glee Hills*. These Sections were constructed by Messrs. 

 Aveline, Selwyn, Bristow, and Ramsay ; and the mapping of the 

 county to which they refer was executed by them and Mr. Jukes. 



Sections Nos. 1 ^ 2. — The oldest rocks crossed by Sections 

 Nos. I & 2 lie at the base of the Merioneth anticlinal of Professor 

 Sedgwick. They are the Barmouth and Harlech sandstones, which 

 are here and there interstratified with beds of purple slate. Their 

 base is not exposed, and the lowest beds that rise in the centre of the 

 area are from 6000 to 7000 feet beneath the base of the Lingula 

 flags. In places they are pierced by numerous greenstone dykes, a 

 few of which are magnetic. No fossils have heretofore been found 

 in them. These and their equivalents are the rocks coloured as 

 *' Gambrian" by the Geological Survey. 



In Section No. 1 the whole of the black and ferruginous slaty and 

 sandy beds that lie between the Barmouth sandstones and the volca- 

 nic ash on the north slopes of Gader Idris belong to the Lingula 

 flags. They are about 7000 feet thick. Here and there, both in 

 the line of section and in the neighbouring areas, masses of greenstone 

 are protruded among them. These generally have a tendency to 

 pass into the lines of bedding, but not unfrequently they cut some- 

 what across the strike, and divaricate into two or more branches. 

 They are therefore known to be intrusive. 



The slaty Lingula flags are succeeded in the ascending station by 

 about 3000 feet of felspathic and calcareous ashes, here and there 

 interstratified with bands of slate, indicatmg the successive accumu- 

 lation of the ashy beds with periods of intermittent repose. They 

 are often conglomeratic and brecciated, and sometimes give the 

 impression that bombs have been shot from the volcano into the air, 

 and fallen among the felspathic dust in a viscous condition. Much 

 of the ash is also porphyritic. The crystals of felspar are always 

 scattered and frequently fragmentary, and may have been showered 

 out along with the volcanic dust and lapilli in the same manner that 

 corresponding phsenomena occur at the present day, it being known 

 that the ashes of existing volcanos are often composed, in great part, 

 of fragmentary crystals. It is said that perfect crystals of augite 

 have been ejected in showers of ashes from Etna. 



Two large masses of intrusive felspathic traps lie between the out- 

 crop of these ashes and the estuary of the Mawddach. The higher 



* The corresponding Horizontal Sections of the Geological Survey are sheets 

 26 & 27 for Section No. 1 ; sheets 28, 29, & 30 for No. 2 ; and sheets 35 & 36 

 for No. 3. 



