TATE: YORKSHIRE PETROLOGY. 
379 
surface shews horn-blende to be entirely wanting. Throughout the 
sections of these dykes these two minerals are so constantly present 
relatively in inverse ratio as to justify the inference that the ingre- 
dients of the biotite have been derived from the bleached hornblende. 
Are these three dykes then related ? The study and comparison 
of an extensive series of slides may enable us to attempt an answer 
to this enquiry. The first slide prepared (in 1879) from near the 
surface of dyke No. 1 was so much decomposed that it became 
necessary to obtain deeper-seated and better-preserved specimens. 
If we arrtinge in groups, say (1) Slides well preserved from dyke 
No. 1 : (2) surface of dyke No. 1 : (3) Best preserved south side of 
dyke No. 3: (4) Slides taken from south to north of dyke No. 2; 
we shall find an insensible gradation in the modifications which the 
essential constituents of the primary hollocrystalline Mica-syenite 
liave undergone. 
The slides of group (1) we have already described as consisting 
of clearly outlined orthoclase, bleached hornblende and fresh biotite. 
Group (2) show decomposing hornblende difficult of identification, 
with relatively more biotite than is found in the previous group. 
The slides of group (3) are devoid of hornblende, and rich in biotite. 
The felspar in each case becomes less and less definite, both in outline 
and colour. In slides of group (4) the individuality of the felspar 
crystals is gone, while of hornblende there is no further trace. 
Finally, the four groups show conclusively that the apparent pseudo- 
morphs after olivine (Figs. 16 and 17), are produced by the decom- 
position of the clusters of bent and separated plates of biotite, as 
seen in transverse sections (Figs. 12 and 13) ; while the appearance 
of a cross-section of zoned nepheline (Figs. 18 and 19), compare 
(Fig. 8, 9, 10, and 11) is simulated by the decomposition products of 
similar plates when view^ed in the direction of the main axis. Inter- 
mediate stages are usually present (Figs. 21, 22, and 23). From 
these petrographical observations we are led to infer, that were we 
to follow these three dykes down to their origin, we should find 
them springing from a common magma. 
