BALDERSTON : THE SILURIAN ROCKS OF INGLETON. 1 39 



dyke up the hill seems to be quite as great, (iii) In the Doe, 36 ft. 

 wide, with 180 ft. of same series between it and No. 3. (iv) In 

 Easegill, 40 yds. from the second bend of the stream above the Foss. 

 This dyke generally has the appearance of a fine freestone, of dirty 

 yellowish-brown colour ; in some stages of change it has a granular 

 character, and when blasted is white, with a pale bluish tint ; it is 

 extremely altered everywhere, but shows its character best in its lava- 

 flow in Easegill, and fairly in the dyke sections, where I have 

 recently blasted the rock both in the Twiss and Doe valleys. The 

 line of exposure is decidedly flexuous or devious, as compared with 

 the felspathic ash, dark limestone, and black cleaved slates in which 

 it is found. In the top of Pecca Hill it is not more than 90 ft. from 

 the Green Slates ; in the river just below it is as much as 270 ft. from 

 the same slates; in the Doe valley the distance is 240 ft.; whilst at 

 Easegill it amounts to not less than 462 ft, and probably 150 ft. 

 more. This dyke is evidently a much -decomposed felstone, 

 consisting of minute granular quartz, pale felspar, hardly dis- 

 tinguishable hornblende, thin scales of black or pale mica, pyrites, 

 and infiltrations through cracks from the black slates, leaving dark 

 stains and patches. The extreme points of exposure are if miles. 

 Rhyolites. — At Easegill, at the base of the tricleaved mudstones 

 which fill a depression in the Upper Silurian sea, are four submarine 

 layers of felstone rhyolite, with a total thickness of about 5 ft., and 

 three thin bands and films of indurated mudstone sediment between. 

 The under surface of the lowest flow has a decidedly brecciated 

 appearance. Some portions of the beds are identical in constitution 

 and degree of change with the Great Dyke itself, but one section 

 yields a compact whitish-grey exposure. Near the top of Pecca Hill 

 the Great Dyke also spreads out into similar lava-flows, separated by 

 shale, and there is not the slightest doubt that this little section 

 of Upper Silurians rests directly and almost horizontally on the 

 practically vertical, black-cleaved slates. 



No. 5. — The Black Dyke, (i) Three exposures south of the 

 Slate Quarries of the Doe. (ii) One south of Skirwith. The width 

 varies from 6 ft. to 3 ft. Three of the exposures, with the exception 

 of the crust, are sound, but the one to the N.W. is much jointed and 

 decomposed, and except at this point there seems to be little varia- 

 tion in the character of the rock. The line of exposure near the 

 Doe, where there are 93 ft. of ash between it and No. 4, is nearly 

 straight, but to the S.E. it seems to have worked nearer to the 

 S. Western edge of the ash bed. This dyke is a divergent granitic 

 trap, approaching very closely kersantite, but has too much ortho- 

 clase present. Constituents : orthoclase, red and pale ; oligoclase, 



May 1889. 



