TATE: YORKSHIRE PETROLOGY. 
313 
felspars ; the small crystals of uniform size diftused through, and 
originally forming the chief component of the ground-mass ; and 
larger crystals, solitary, or grouped in clusters, porphyritically em- 
bedded in the ground-mass to some extent, in fact, repeating the 
" glomero-porphyritic" structure of Prof. Judd. Each of these 
glomero-porphyritic clusters is enclosed in a framework of secondary 
magnesian mica generated around it. The external crystal- 
line faces of the felspars from all tlie cluster members of this 
group of dykes are well-nigh obliterated, not being at all so definite 
as those of the clusters in Phillips's dyke ; but the internal boun- 
daries have been from the first, in both cases, irregular and ill- 
defined. ^J^he result is that while the clusters in this dyke have 
succumbed to degrading influences even to the limit of effacement, 
the similar clusters in Phillips's dyke having resisted corrosive 
agencies most effectually, present to us the freshest felspars yet met 
with in these Lamprophyres. Of the characteristic twin-lamellation 
of plagioclase felspars there is no trace ; and the little that remains 
in evidence, whether of form or colour, favours orthoclase, as analogy 
would suggest. The orange-red particles of the original felspar are 
by no means so abundant as we found them to be in Phillips's dyke. 
They are accompanied here and there by a cumulous aggregate of 
minute granules of opaque white matter, probably kaolinite, and iu 
some examples the nature of the felspar is completely masked by 
kaolinisation. If we select one of the least altered, it may show 
kaolin traversing cleavage planes (fig. 33), a process, which when 
more advanced, and in the glomero-porphyritic clusters, produces 
effects obscurely simulative of olivine pseudomorphs, a suggestion 
however negatived, by reflected illumination revealing surviving 
patches of pink felspar (figs. 34, 35. 36). These figures may be com- 
pared with those of similar effects noticed in the white dyke of Dale 
beck (figs. 16-17), and which may have been fashioned in like manner, 
no primal component having survived to marshal us the way we 
ought to go. 
The nature of the original ferro-magnesian ingredient of this 
dyke is no longer determinable. The micro-granular greenish-grey 
material diffused throughout the slides has iu one instance retained 
