320 J. ARTHUR THILLIPS ON THE ROCKS OF 



" Elvan-courses," or dykes of granite or of quartz-porphyry, 

 not only traverse the granites and slates, but also pass indiscri- 

 minately through all the other rocks of the mining districts. Their 

 direction is most commonly to the north of east and south of west ; 

 but there are numerous exceptions to this rule. In width they vary 

 from a few feet to many fathoms. As these dykes intersect the 

 granite, which itself sends off veins into the killas, it is evident that 

 their protrusion must have taken place subsequently to the consoli- 

 dation of the former rock. 



In addition to the protrusion of granite in Post-Carboniferous times, 

 and the subsequent ejection of elvans, evidences abound of active 

 volcanic action, and of the outpouring of enormous volumes of igneous 

 matter during the deposition of the Devonian rocks. These had 

 experienced a large amount of upheaval and disruption prior to the 

 deposition of the Carboniferous series. Many of these rocks are 

 doleritic lavas, and do not materially" differ from those which have 

 issued during Tertiary times from the craters of more modern 

 volcanoes*. 



Intrusive dykes of diorite &c. are met with in various parts of the 

 county ; but many of the " greenstones " marked on the Ordnance 

 Geological Map are unquestionably metamorphosed slates. A con- 

 siderable area of serpentine and diallage rock occurs in the Lizard 

 district ; and serpentine and diallage are also found in the neighbour- 

 hood of St. Keverne. 



Both the sedimentary and igneous rocks of Cornwall are traversed 

 by innumerable mineral veins, which, although principally composed 

 of siliceous materials, contain ores of tin, copper, lead, and various 

 other metals. Veins yielding ores of tin and copper have usually a 

 direction approximating to east and west, and are seldom found at 

 any considerable distance from the junction of the granite and killas, 

 particularly if elvan-courses do not occur in the neighbourhood. 

 These veins, which are called " lodes," are intersected, nearly at 

 right angles, by others known as " cross veins," which sometimes 

 yield lead or iron ores, but are otherwise, excepting in the immediate 

 vicinity of lodes, usually unproductive. 



Veins having a somewhat similar direction to that of true lodes, 

 but which intersect them at a more or less acute angle, are called 

 " caunter-lodes," and are not imf requently metalliferous. 



Killas. 



The clay-slate resting upon the flanks of the several granite- 

 masses of Cornwall, and partially filling up. the hollows between 



* There is. in fact so intimate a mixture of compact and trappean rocks with 

 the argillaceous slates, that the whole may be regarded, as one system, the two 

 kinds of trappean rock having probably been erupted, one in the state of igneous 

 fusion and the other in that of ash, during the -time that the mud now forming . 

 slates was deposited, the mixtures being irregular, from the irregular action of 

 the respective causes which produced them ; so that though the one may have 

 been derived from igneous action, and the other from the ordinary abrasion of 

 pre-existing solid rocks, they were geologically contemporaneous. (Dela Beche, 

 ' Eeport on the Geology of Cornwall, Devon, and West Somerset,' p. 57.) 



