82 ME. R. H. WORTH OX THE [vol. lxXV, 



The Later Aluminous Rocks. 



Reference has been niade to the aluminous shale which conies in 

 to the north of the calcareous series, and is to be found in the 

 Railway Quarry. At first sight, this would appear to be again 

 succeeded by calcareous shales, but the distortion at the eastern 

 end of the quarry is possibly the cause of this apparent succession. 



The rock in question is blue-grey, almost black, somewhat soft 

 when quarried, but hardening on exposure. It is crowded with 

 chiastolite-laths, which are not very clearly visible on fresh speci- 

 mens. The transverse sections of these laths frequently show true 

 crosses, with four external re-entrant angles, and are sometimes 

 even more complex. The rock itself is traversed by close-set, 

 parallel, opaque lines, with small transparent ' eyes ' set between 

 them ; this structure is continued right into the re-entrant angles 

 of the chiastolite, and where, as is frequently the case, the outer 

 skin of the crystal is altered to mica, the mica forms a clear-cut 

 boundary to the cross (see PL V, fig. 1 : LXXVI, S.E. 57). 



This chiastolite-rock is, on the surface, some considerable dis- 

 tance from the granite, rather more than a mile, and the question 

 suggests itself whether the mineral is due to the rock lying within 

 the metamorphic aureole of the granite, or is a feature of contact- 

 metamorphism induced by the veins of ' dark igneous ' rock which 

 occur in the same quarry. 



IV. The Calcareous Series. 



With few exceptions, the rocks of this series are either porcel- 

 lanous, calc-flinta, or chert-like. They vary in colour from white 

 to black, and frequently show a well-marked conchoidal fracture. 



The exceptions are, in the first place, the Meldon Limestone, 

 and in the second place certain beds which have decomposed into 

 either black or dark purple-brown clays. The purple-brown clays 

 may occur anywhere in theporcellanous series, but rarely form any- 

 thing approaching continuous sheets. Transition-forms of partly 

 decomposed rock are found, and these make it evident that the 

 whitest of the rocks may yield this decomposition -product. The 

 black clays are found between chert-like beds near the limestone ; 

 they appear to form continuous sheets, and are well exposed on the 

 eastern bank of the AVest Okement, opposite the Limestone Quarry, 

 and close to the compressor-house of the Meldon Valleys Company. 



Black chert-like rocks are associated with the limestone, and 

 occur also at Sourton Lime-Quarry, 3 miles away to the south- 

 west. Two other belts of apparently similar rock exist. The one 

 at LXXVI, S.E. 11, on the banks of the Redaven, where a pit has 

 been sunk upon it, and the other as a prominent outcrop at 

 LXXVI, S.E. 20, some 1100 feet or so away to the east. 



It was probably the presence of this rock at 11 which led 

 De la Beche to map a second outcrop of the Meldon limestone ; it 

 also induced would-be workers of lime to sink the pit, and to 



