720 KBPORT — 1887. 



It is very probable tliat the Keuper basement beds of the Porlock valley may 

 be marprinal deposits formed during a progressive subsidence, and therefore may 

 belong to a higlier horizon than the Lower Keuper beds south of Williton. 



2. The Devonian Bodes of West Somerset on the Borders of the Trias. 

 By W. A. E. UssHER, F.G.S. 



The composition of the Quantacks is first briefly described, and the faulted 

 relations of the middle Devonian grits, slates, and limestones of which they con- 

 sist alluded to. From the constitution of the Palpeozoic districts on the east and 

 west of the Triassic rocks of Crowcombe and Stogumber the author considered the 

 beds eroded in the intervening valley would amply account for the variability of 

 the Triassic strata derived from them. From Withycombe to Porlock the faulted 

 relations of the middle and lower Devonian grits are then briefly described. The 

 author considered that the elevation of the Quantocks, the Brendon, and the Dun- 

 kery ranges was pre-Triassic, accompanied by faulting on an extensive scale ; that 

 many lesser faults were produced in post-Triassic times, and that further movements 

 took place along the old lines of fracture. He did not believe that the Devonian 

 highlands were ever covered by secondary sediments, but was of opinion that the 

 Triassic rocks never extended far beyond their present boundaries, except in old 

 valleys from which they had subsequently been almost entirely removed by de- 

 nuding agencies. 



3. The Matrix of the Diamond. By Professor H. Carvill Lewis. 



A microscopical study of the remarkable porphyritic peridotite which contains 

 the diamonds in South Africa demonstrates several interesting and peculiar features. 



The olivine, forming much the most abundant constituent, is in porphyritic 

 crystals, sometimes well bounded by crystal faces, at other times rounded and with 

 corrosive cavities, such as occur in it in basaltic rocks. It rarely encloses rounded 

 grains of glassy bronzlte, as has been observed in meteorites. The olivine alters 

 either into serpentine in the oi-dinary way, or into an aggregate of acicular tremo- 

 lite crystals, the so-called ' pilif,'' or becomes surrounded by a zone of indigo blue 

 bastite — a new variety of that substance. The oUvine is distinguished by an 

 unusually good cleavage in two directions. 



Bronzite, Chrome dialhiije, and Smaragdite occur in fine green plates, closely 

 resembling one another. The bronzite is often surrounded by a remarkable zone, 

 •with a centric, pegmatitic, or chondritic structure, such as occurs in certain meteo- 

 rites. This zone is mainly composed of wormlike olivine grains, but a mineral 

 having the optical characters of cyanite also occurs in this zone. 



JJiotite, ii characteristic constituent, occurs in conspicuous plates, often twinned, 

 generally rounded, and distinguished by its weak pleochroism, a cliaracter pecu- 

 liar to the biotite of idtra-basic eruptive rocks. It alters by decomposition into the 

 so-called Vaalite. 



Ferorskite occurs in very numerous but small crystals, which optically appear to 

 be compound rhombic twins. 



Pyrope is abundant in rounded red grains. Titanic iron, chromic iron, and 

 some fifteen other minerals were also found. Rutile is formed as a secondary 

 mineral through the alteration of olivine into sei'pentine, being a genesis of rutile 

 not heretofore observed. 



The chemical comjwsition shows this to be one of the most basic rocks known, 

 and is a composition which by calculation would belong to a rock composed of 

 equal parts of olivine and serpentine, impregnated by calcite. 



The structure is at the same time porphyritic and brecciated, being one charac- 

 teristic of a volcanic rock which after becoming hard had been subjected to 

 mechanical movements. It is a volcanic breccia, but not an ash or tufl^, the peculiar 

 structure being apparently due to successive paroxysmal eruptions. A similar 

 structure is known in meteorites, with which bodies this rock has several analogies. 

 A large amount of the adjoining bituminous shale is enclosed, and has been more 



