. The Quarterly journal of the Geological Society of London. uth-western end of theoutcrop. Here the intrusion is quite thin, measuring not more than15 or 20 feet in thickness, and is wedging out westwards amongthe sandstones. Both contacts, bordered by hardened white sand-stones, are seen. The marginal facies is a decomposed brownishaphanitic rock showing a few flakes of biotite. The interiorconsists of decomposed teschenite, but there is no trace of theother facies. A mass of theralite probably connected with the Lugar sill, butwith a distinct dyke habit, crosses the Lugar Water in a north t
. The Quarterly journal of the Geological Society of London. uth-western end of theoutcrop. Here the intrusion is quite thin, measuring not more than15 or 20 feet in thickness, and is wedging out westwards amongthe sandstones. Both contacts, bordered by hardened white sand-stones, are seen. The marginal facies is a decomposed brownishaphanitic rock showing a few flakes of biotite. The interiorconsists of decomposed teschenite, but there is no trace of theother facies. A mass of theralite probably connected with the Lugar sill, butwith a distinct dyke habit, crosses the Lugar Water in a north tosouth direction, about 250 yards west of Logan Bridge. Thecontacts are not seen ; but, on the northern bank of the river, therock forms a small knoll with vertical sides, and has the aspect of a ?dyke. In appearance and microscopic structure it is identical with,the dominant phase of the theralite stratum in the Lugar sill. Q. J. G. S. No. 286. i 94 ME. Gt. W. TYRRELL OK THE [vol. lxxii, Fig. 3.— Geological map and section of a part oftJie Lugar [P = Picrite and peridotite ; T = Teschenite; Th. = Theralite.](4) Summary. It is now possible to attempt a general view o£ the Lugar sill,and of the structure and disposition of the various facies of whichit is composed. This mass, 140 feet thick, was intruded into coldrocks, as testified by the chilled contacts at both upper and lowermargins. The upper and lower contact-basalts are identical incomposition. The chilling influence of the contact is manifest infineness of grain to a maximum thickness of 10 feet, after whichthe rock passes rapidly into a coarse teschenite at both margins. part 2] PICRITE-TESCHE1UTE SILL OP LUGAR. 95 A movement of the magma has given rise to distinct schlieren,distinguishable by slight differences of colour and texture, at bothcontacts. The same explanation can hardly be applied, however,to the remarkable differences obtaining in the interior of the the contact-basalts and the nor
Size: 1364px × 1831px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
