464 PKOE. W. S. BOULTON ON A MONCHIQUITE INTRUSION [Nov. I9II, 



direction, and it may be a dyke of considerable width, which ends 

 abruptly at the south-eastern end where the quarry has been lately 

 worked; and, after for a time traversing the sandstones roughly 

 parallel with the bedding, the magma may have passed upwards 

 into strata now removed. No trace of the igneous rock, however, 

 either in outcrop or in surface-fragments, could be seen along this 

 crest, west of the old quarry already mentioned. 



If the junction of the monchiquite and sandstone at the eastern 

 end of the recent quarry is to be regarded as one of the walls of 

 a dyke, then the dyke would trend east-north-eastwards, which is 

 the direction of the Bartestree dyke, near Hereford (mentioned on 

 p. 474). But if so, it would be of great thickness, between 200 

 and 300 feet ; and moreover, the form of the ground, which is 

 probably determined to a great extent by the very hard igneous 

 rock, is against the dyke having this direction. 



The plentiful inclusion of xenoliths of the country-rock is not 

 inconsistent with the view that the monchiquite is a dyke or a sill 1 ; 

 but, having regard to the shape of the known outcrop, it seems not 

 unreasonable to conclude that we have here a volcanic plug or 

 bysinalith 2 with an irregular sill-like extension in an easterly 

 direction. 



The available evidence, however, is by no means conclusive, and 

 the exact nature of the intrusion must be regarded for the present 

 as doubtful. 



III. Peteograf-hical Details. 



Owing to the extent to which the rock has been weathered, and 

 to the fact that the quarry is not being worked at the present time, 

 big fresh lumps are difficult to obtain. Then, too, the included 

 Old-Bed-Sandstone fragments are very plentiful in places ; and, as 

 will be shown later, the monchiquite has an abnormal development 

 of carbonate and other secondary products in their vicinity. 



"Where the rock is coarsely porphyritic, as in the central parts of 

 the quarry, the large augite and biotite-crystals arrest attention. 



Augite. — Perhaps the most striking feature of the rock is the 

 unusual development of the augite-phenocrysts. An individual 

 crystal was measured, having a length of 6 inches and breadth 

 of 3 inches, while prisms 3 or 4 inches long are quite common. 3 

 Occasionally they have their original crystalline form in part well 

 defined, but usually the crystals have suffered much corrosion, and 

 many are quite spherical or ovoid in shape with no trace of their 

 original form, and in not a few cases they are deeply embayed by 

 the matrix, or almost entirely resorbed. 



1 See A. Harker, ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 

 1904, chap. xx. 



2 J. P. Iddings, ' Igneous Eocks ' vol. i (1909) p. 315. 



3 No evidence is forthcoming that these large augite-phenocrysts are aggre- 

 gates, producing a glomeroporphyritic structure ; in all cases they 

 appear to be single crystals. They are, again, easily distinguished in the field, 

 and under the microscope, from the xenoliths of picrite described on p. 471. 



