SOME NORTH-OF-ENGLAND DYKES. D5 
mann among the road-metal at Hartlepool; it is an angular fragment 
of what is now a light-grey quartzite. The sharpness of the angles 
of this specimen appears remarkable when one takes into considera- 
tion the evidence of extreme metamorphism which the microscope 
affords. 
The inclusion, as a whole, is now composed of extremely irregular 
grains of crystalline quartz, which fit together so as to leave no 
interspaces. Here and there, however, more or less oval spaces 
occupied by the magma of the igneous rock may be recognized, and 
in much of the quartz secondary glass inclusions similar to those 
described by Chrustschoff* and Dolter tf occur. 
The other macroscopically recognizable constituents are blebs of 
quartz (Ayton, Preston, and Cockfield), small agates, usually 
associated with calcite (Bowles Beck), and irregular patches of 
pyrites. These, however, are not uniformly present, but occur 
rather as local peculiarities. 
I have made 23 specific-gravity determinations on specimens 
‘taken from widely separated localities, including Great Ayton, 
Preston, Bolam, Cockfield, Bowles Beck, Tyne Head, and Arma- 
thwaite, with this general result that, when care is taken to select 
unaltered or but slightly altered specimens, the specific gravity is 
found to lie between 2°765 and 2-788. The best observations 
indicate 2°77 as the mean. Altered specimens have a lower specific 
gravity: thus two from Cockfield and Tyne Head respectively gave 
2°63 and 2°62. 
The microscopic as well as the macroscopic characters are remark- 
ably constant along the whole length of the dyke, so that in 
describing the rock from a microscopic point of view it will not be 
necessary, except in a few special cases, to refer to separate localities. 
Two important structural types may be observed ; the one character- 
istic of the main mass of the dyke, the other of the few inches 
which form the margin. These will be referred to again when the 
separate constituents have been described. 
The original constituents may be considered under the following 
heads :—(a) porphyritic felspars, (>) felspars of the ground-mass, 
(c) augite. (d) magnetite and ilmenite (?), (¢) biotite, (f) apatite, (g) 
interstitial matter with globulites, longulites, microlites, &e. 
The two varieties of felspar will be referred to simply as the large 
and small felspars; they belong, of course, on the whole to two 
different stages in the process of consolidation, although it is probable 
that the outer zone of the largefelspars was frequently added during 
the formation of the felspars of the ground-mass. Biotite occurs 
sparingly, and has only been observed in certain sections from Ayton 
and in one from Armathwaite. Apatite does not seem to be 
abundant ; indeed its presence is inferred rather from the chemical 
than from the microscopic analysis. By interstitial matter is meant 
* Chrustschoff, “Ueber secundare Glaseinschliisse in den Gemengtheilen 
ae -Gesteine,” Tschermak, Min. u. petr. Mitth. 1882. Neue Folge iv. 
p- 4 
+ Dolter and Hussak, “Ueber die Hinwirkung geschmolzener Magmen auf 
verschiedene Mineralien,” Neues Jahrbuch, 1884. Bd. i. p. 18. 
