462 PEOF. W. S. BOULTOtf ON" A MONCHIQUITE INTRUSION [Nov. I9II, 



from the augites. They are made up of a confused aggregate of 

 a brown or green, bronzy looking mineral, with a marked mica-like 

 cleavage ; together with a purplish-grey substance that effervesces 

 under acid ; small, irregular, black crystalline grains of augite ; and 

 much green and yellow colouring-matter. As will be shown later, 

 these are included lumps of olivine-augite rock, distinct from, but 

 related to, the monchiquite in which they occur. 



A very noticeable feature is the large amount of the country- 

 rock that has been caught up by the intruded magma. Chips of 

 sandstone and bigger lumps, upwards of 2 feet across, both of 

 sandstones and marl, are irregularly distributed — in one part of the 

 mass so thickly as to suggest a tuff or agglomerate ; and on the 

 weathered surface they give to the rock a very rough, rusty looking 

 appearance. 



The included lumps of marl are generally well rounded, and 

 have been converted into a dark porcellanite with a sharp flinty 

 fracture ; while the subangular lumps of sandstone have a bleached 

 -appearance or yellowish-brown colour, and break with a lustrous 

 fracture. Much secondary calcite, silica, and chloritic matter occur 

 in the vicinity of these xenoliths, the silica often assuming an agate- 

 like disposition. 



On the northern face of the quarry, particularly in the central 

 jpart, the rock shows a pronounced spheroidal jointing, with the 

 .characteristic weathering into concentric shells. 



The principal linear joints run in two directions, one set nearly 

 due north and south, and the other north-west and south-east. 

 Iron-stained silica, with some calcite and chlorite, and a little 

 earthy manganese oxide and copper carbonate fill most of the joints 

 and cracks. 



The nature of the intrusion and its relation to the 

 ■surrounding rocks are difficult to determine. Only at one place 

 is it possible to see a junction of the igneous rock and the sandstones : 

 .namely, at the eastern side of the quarry. Here the invading rock 

 • cuts obliquely across the sandstones, which are dipping at this 

 spot at about 30° north-eastwards, 1 so that the line of junction is 

 ^roughly in an east-north-easterly direction. 



The monchiquite, which is exposed at the centre of the quarry to 

 ~a depth of some 6 feet below the level of the sandstone, abuts against 

 the sandstone with a nearly vertical junction-surface, running 

 with the dip. The junction then coincides with the bedding in a sill- 

 like way, for about 12 or 15 yards to the end of the quarry. .For 

 some 3 or 4 feet from the junction the igneous rock assumes a less 

 massive appearance ; it has a rough, scoriaceous, and finely nodular 

 texture with a coarse pseudo-lamination, the joint-planes lying 

 nearly vertical. In this shaly-looking igneous rock lie rounded 

 lumps of burnt marl, measuring from 6 inches to 3 feet across. 

 They are brown and yellow on the outer burnt border, but dark 



1 The general dip hereabouts is 10° east-north-eastwards. 



