140 MR. J. MORRISON OX [vol. lxxiv, 



and less basic derivatives of a magma practically free from ready- 

 made felspars, a period of renewed activity set in over a wider area 

 resulting in a set of intrusions which, while sometimes availing 

 themselves of former lines of weakness (forming thereby composite 

 dykes), overlapped in every direction the former scene of activity 

 and gave rise to the numerous lamprophyres and felsites of the 

 ■outer parts of the area. As the earlier set of intrusions coincide 

 generally in direction with the series of north-north-west faults, 

 so many of the later set coincide in direction with the series of 

 faults ranging from north-east to north, fracturing the country 

 east of the Lune. The two distinct types of intrusion thus 

 appear to be related to the two main types of fracture of the 

 district, and it is of interest to note the order in which the two 

 were intruded. 



Some dvkes in Long Sleddale Valley throw light on the closing 

 stages of activity. Two were described by Dr. A. Harker 1 in 1912, 

 and the incorporation of acid rock by basic was especially noted. 

 Two features of interest arise from their study. The upper dyke 

 is a lamprophyre of the minette-kersantite type, consisting essenti- 

 ally of felspar and biotite with some hornblende and magnetite. 

 Enclosed are portions of a felsitic rock, mainly of orthoclase 

 and quartz with much secondary material ; but the quartz is 

 •often anterior to the felspar, and sphene is present, two features 

 linking it with the granite. The lower dyke contains similar 

 Enclosures, presumably from the same source ; but the dyke itself 

 is more basic. Plagioclase and hornblende are the essential con- 

 stituents, the remaining minerals including biotite, apatite, magne- 

 tite, and irregular pseudomorphs of calcite and chlorite. The rock 

 is thus allied to the spessartites. A somewhat similar dyke in 

 ■contact with the upper lamprophyre presents a chilled edge to 

 it. The specific gravity of the spessartite is 2 - 729, and that of 

 the earlier intrusion 2*665. 



Whether the acid inclusions were brought up in a fluid state, or 

 were picked up by the lamprophyres during intrusion, cannot 

 be definitely stated ; but the fact that the smaller patches some- 

 times show flow-structure with the linear arrangement parallel 

 to the walls of the dyke, shows that the enclosed rock was suffi- 

 ciently plastic to be moulded, and as analogous features are present 

 in some of the dark pitches of the granite, the two may be com- 

 parable in this respect. This would indicate that the two magmas 

 co-existed in a fluid state, the result of differentiation on a wide 

 scale, of which the central portion of the composite dyke in 

 Stakeley may be a product. At what stage the advanced differentia- 

 tion occurred it is impossible to state ; but its manifestation was 

 considerably later than the intrusion of the mixed series, and was 

 probably connected with the movements which caused a swinging 

 of the later products of the deep-seated magma from a south- 

 easterly to a south-westerly direction. In the adjoining area 



1 * The Naturalist ' 1912. pp. 2GC. <',7. 



