part 2] THE SHAP MIXOH INTBUSIONS. \X>' 



of the dark rock involved in the How of the acid magma. Los. 

 than 12 yards away the sharp junction is absent, and no included 

 blocks are found; but there is apparently a gradual transition from 

 one to the other. 



Higher up the same valley is another composite dyke, the study 

 of which yields results of considerable importance. One of the 

 members is a typical quartz -porphyry containing phenocrvsts of 

 quartz, microperthite, oligoclase. and occasional mica in a pink 

 ground-mass, compact and felspathic in the hand-specimen, finely 

 microspherulitie in thin section. Biotitc is absent from the 

 ground-mass, which consists almost entirely of orthoclase and 

 quartz. This part of the intrusion contains no large granitic 

 orthoclases, and, with its scarcity of biotite and abundance of 

 quartz, is more acid than any of the orthoporphyritic dykes. 

 Alongside conies a dark, more basic rock (evidently of the mixed 

 series) in which pink orthoclase-xenocrysts occur only at the- 

 margin. At first sight, the whole appears to form one intrusion ; 

 but closer inspection shows that the central portion, which is- 

 entirely free from felspar-xenocrysts, is a later product of the 

 magma intruded along the same channel. The junction, when 

 detected, is quite sharp, and the chilled edge is conclusive. A 

 comparison between the two rocks is instructive. The earlier 

 marginal rock is a modified lamprophyre of the mixed series differ- 

 ing in no way essentially from that of the composite dyke at Gill. 

 The central intrusion, however, is distinguished by the absence of 

 felspar-xenocrysts, though rounded quartz is present. It is a dark- 

 grey porphyritic rock ; but the phenocrvsts are of a small order,. 

 yellowish-green oligoclase being the most common, while biotite is- 

 perhaps not quite so abundant as in the marginal intrusion. In 

 thin section biotite and felspar are again the chief constituents; but 

 the texture is almost trachvtic, the ground-mass consisting largelv 

 of laths of striated felspar and prisms of orthoclase, with some- 

 quartz, sphene, apatite, and pyrrhotite. The rock appears to be a 

 transitional variety resembling in some respects a lamprophyre, 

 in others a porphyrite. The abundance of oligoclase-phenoerysts- 

 and the diminution in biotite determine its classification as a 

 biotite-porphyrite. On the whole, therefore, it is less basic than 

 the earlier marginal rock, and this is borne out by its specific 

 gravity: 2'680 as compared with 2*713. The effect on the earlier 

 rock has been remarkably small, the only change recognizable in 

 section being the production of biotite in the pseudomorphs. 



Consideration of this central member raises the question of its 

 magmatic source. If it belongs to the same period as the mixed 

 series of intrusions and is derived from the same magma, then the 

 absence of the orthoclase-xenocrysts would indicate a lower horizon 

 than those that furnished material for any known member of the 

 series, and consequently one would expect to find a further in- 

 crease in basicity. The reverse being the case, it is evidently not a 

 product of the mixed magma ; and it must be assigned to a still later 

 period, when, differentiation on a large scale having produced more 



