A Geological Description of the Parish of Portisheud. 263 



^damp as to be unfit for building, this rock does not give origin to springs, 

 but, on the contrary, often engulfs them, when they flow upon it from the 

 old red. 



3. The pennant is usually separated from the carboniferous limestone 

 by a cousiderable thickness of millstone, or Brandon Hill, grit and quartz 

 conglomerate. These rocks are, however, almost if not altogether wanting 

 in this district, in which the pennant lies immediately upon the limestone. 

 The pennant is a sandstone more or less fissile, and varying in appearance ; 

 it contains a great variety of fossils, chiefly tropical plants, and it passes 

 gradually into the rocks containing tlie coal, or coal measures. 



II. The new red sandstone formation is composed of three members : — 

 1. the doloraitic or magnesian conglomerate j 2. the new red sandstone j 3. 

 upon the summit, the red marl. 



1. The dolomitic conglomerate is composed of pebbles from various older 

 rocks, cemented by a magnesian paste. These pebbles vary according to 

 the structure of the rocks upon which the conglomerate rests, and are in. 

 quantity also extremely irregular. Sometimes there are no pebbles ; and, 

 as on the coast of Sully, the rock is perfectly homogeneous. Such a rock, 

 if its origin be considered, is not likely to contain many very perfect fossils. 

 At Portishead it contains a few crinoidal vertebrae, but even these are rare. 



2. The new red sandstone, overlying the conglomerate, is in its appear- 

 ance extremely variable ; it is usually of a deep red hue, whence its name ; 

 but at Portishead, it is of a fine bright buff colour. It contains scarcely 

 any fossils, and those only in minute portions. 



3. The red marl, degrading into a red soil, forms the upper member 

 of the series, and is often of great thickness. At Portishead it is only ob- 

 served in a very minute quantity j it contains chrystals of strontian and 

 other minerals, but no fossils. 



Such being the general characters of the rocks found in the district we 

 have selected on the present occasion, we shall now proceed with the de- 

 tails of our description to some minuteness, and then enumerate the prin- 

 cipal facts observed, and the inferences obviously deducible from them. 



The mill at the head of the creek stands upon alluvion and made ground, 

 but the red marl is seen in the bank of the road just west of it. Ascending 

 the hill through the new lodge, is the new red sandstone, not very well seen ; 

 but higher up, the hill is masked with the debris of old red sandstone of a 

 thickness of eight or ten feet; and towards the summit the rock itself 

 appears, and is quarried extensively. It there dips 17", and bears E. by 

 N. true bearing. 



These beds are highly micaceous, fissile, abounding in indurated clay 

 nodules, and contain a very few quartz pcbijlcs. The mica has a green 

 tinge. 'J"he beds vary from one to ten feet in thickness, and arc quarried for 

 flags and building stone. The quarry is also employed as a stone-mason's 

 yard for dressing Welsh flags and other stone. The new houses west of 



