474 PROF. W. S. BOULT >N ON A MONCHIQ.UITE INTRUSION [Nov. I9II, 



The conclusion would appear reasonable, therefore, that the two 

 rocks — the included picrite aud the enclosing monchiquite — are 

 nearly related. Unlike the xenoliths of Old Red Sandstone, which 

 are 'accidental,' these picritic nodules must be regarded as ' cognate/ 1 

 and they probably represent portions of the already consolidated, 

 deep-seated magma from which the monchiquite had its source, 

 portions which were carried upwards by the latter in its violent 

 intrusion into its present position. 2 



YI. Affinities with other Rocks. 



When a hand-specimen of the rock, with conspicuous crystals of 

 augite and biotite, was shown to Dr. Elett and Mr. H. H. Thomas, 

 they at once recognized its striking resemblance to some of the 

 basic lamprophyres of Colonsay, recently described by the Officers 

 of the Geological Survey in a memoir now passing through the 

 press. Through the courtesy of the Director and Dr. Plett, I have 

 been able to look over some of the proofs of this memoir, and I have 

 also examined some of the typical rock-specimens and microscope- 

 slides described therein. 3 



Among the minor intrusions of Colonsay, one in particular 

 closely resembles the Monmouthshire rock in many respects. It 

 has the same black, glassy look, with small pink and white ocelli 

 and with large corroded phenocrysts of augite and biotite. There 

 are minor differences : thus the Colonsay rock contains hornblende- 

 phenocrysts and no olivine, and the biotite is more abundant in 

 the ground-mass, while the rock on the whole is somewhat fresher 

 than the Monmouthshire rock. Dr. Flett classes it with the 

 4 ouachitites ' of J. F. Williams. 4 



Except for the higher alkalies, it will be seen (p. 470) that this 

 Colonsay rock is very closely allied chemically to the Monmouth- 

 shire rock. The associated analcite-bearing dolerites (crinanites) 

 of Colonsay and the camptonites of Argyllshire, especially the 

 Ardmucknish rock, the analysis of which is quoted on p. 470, are 

 also very similar in composition. So, too, are the monchiquites of 

 America, described by Pirsson, Weed, and others : an analysis of 

 one of these American rocks appears in the last column on p. 470. 



It becomes important to enquire whether the basic intrusion 

 into the Old Red Sandstone at Bartestree, near Hereford, 5 some 

 30 miles north-north-east of the Monmouthshire monchiquite, and 

 the only other igneous intrusion in the Old Red Sandstone of the 

 South Wales area, is in any way related to the rock which we are 

 now considering. Unfortunately, no analysis of the Bartestree rock 



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

 1904, p. 351, and Journ. Geol. Chicago, vol. viii (1900) p. 394. 



2 ' The Tertiary Igneous Eocks of Skye ' op. cit. p. 362. 



3 The memoir has been published since the above was written. 



4 'The Igneous Eocks of Arkansas' Ann. Eep. Geol. Surv. Arkansas for 

 1890, vol. ii (1891) p. 393. 



5 S. H. Eeynolds, ' The Basic Intrusion of Bartestree' Q. J. G. S. vol. lxiv 

 (1908) p. 501. 



