262 OBSERVATIONS ON THE STRUCTURE OF ROWLEY RAG. 
have no need to say more than that the constituent minerals 
are augite, a triclinic felspar, olivine and its decomposition 
products, with apatite, magnetite, and ilmenite subordinate 
in quantity but almost never failing in any part of the mass. 
In a number of thin sections which I have from time to 
time prepared, I have, however, happened upon a few points 
of rather special interest relating to the structure of the 
mass in different parts; points, too, which have been attract¬ 
ing a good share of attention during the last few months as 
throwing some light on the causes which have brought about 
the curious changes in mineral character among the products 
of volcanic action in many districts when this is continued 
over long periods of time. 
In one or two of my specimens there is a distinctly 
microporphyritic structure to be observed; large felspar 
crystals are scattered through a ground composed of very 
small crystals of felspar and augite, with a very large quantity 
of magnetite disseminated through the mass. This large 
quantity of magnetite is also found in a specimen from 
Tansley Hill, near Dudley, where the minute felspar crystals 
in a few places show most characteristic signs of the mass 
having been in motion after they were formed. * They lie in 
streams, their lengths being to a good extent parallel to each 
other, while the grains of augite are of very small dimen¬ 
sions indeed. 
There are very curious differences hi the texture and 
composition of portions of the mass even close to each other. 
In the space of a circle of one inch in diameter, sudden 
changes from very coarse grain to quite fine may be observed. 
In some sections there is no trace of olivine, in others it 
forms a very important part ; apatite is usually much more 
abundant in the coarse-grained parts than anywhere else ; 
augite sometimes occurs in well-defined crystals showing 
eight-sided sections; sometimes it only fills up the spaces 
between the long blades of felspar. 
I have examined the felspar by Szabo’s flame reactions 
and find that the specimens tried were labradorite, but, of 
course, this does not exclude the possibility of the presence 
of other varieties. 
Mr. Allport, in the paper mentioned above, describes 
certain red veins or masses, “ evidently contemporaneous,” 
as occurring in the midst of the black stone, and it is to 
these and certain grey veins which prove to be closely related 
to the red ones that I particularly wish to call your attention 
this evening. In a recent visit to the Hailstone Hill quarry 
I was fortunate enough to obtain specimens of one of these 
