378 
TATE: YORKSHIRE PETROLOGY. 
(3) If the strike of the above dyke be prolonged under Manor 
House to Dale Beck it connects with a third Mica-trap dyke about 
20 yards lower down that stream than our dyke No. 1, running 
parallel therewith, and having like it, a slight hade to the south. 
The exposed rock mass is about 15 feet long, 8 feet in height, and 
5 or 6 feet in breadth, and it is interesting as being the most 
southerly exposure known of Mica- trap in the British Islands. This 
dyke was extremely puzzling, especially along its northern boundary, 
where it presents an appearance which, for a long time, was 
interpreted as that of a decomposed spherulitic rhyolite, which 
indeed in numerous hand specimens collected at various times it 
closely resembles: flow-structure; impure Kaolin alternating with 
chalcedony in concentric layers or envelopes enclosing a central black 
degradation product, with other indications all pointing to this 
conclusion. 
But the microscopic appearance (Pig. 20) does not confirm, nor 
is the silica sufficiently abundant to sustain this reading ; and indeed 
repeated surveys of the junction in situ have sufficed to demonstrate 
that considerable reaction has taken place between the dyke and the 
contiguous shales, the mutual interchange obliterating the line of 
contact, along most of the exposure, while at one or two points where 
it remains visible it is extremely irregular and eroded, the two 
having eaten into each other persistently. The simulation of 
spherulitic structure alluded to, is due to the fact that the highly 
contorted gnarled effects, superimposed upon the original bedding 
planes, being more or less retained have formed, as it were, an initial 
path along which the alteration of the shales has travelled. 
This trap both macro- and microscopically, a little way from its 
northern boundary, is identical with our dyke No. 2, showing that 
the latter also owes its alteration to the interaction of the trap and 
the contiguous shales. Where the removal of the biotite has been 
completed the rock is vesicular, but along its southern margin it is 
less decomposed. Some twelve feet from the stream this face 
resembles the contiguous dyke No. 1, that is, it consists of an intimate 
mixture of red-felspar and black-mica, the latter much more abundant 
than we found it in our first dyke: but on the other hand a polished 
