﻿14 MISS M. K. HESLOP AND DR. J. A. SMTTHE ON THE [Feb. I9IO, 



Compared with the dykes previously described, the one at Tyne- 

 mouth differs chiefly in having* more dark glass and less augite (and 

 no twisted crystals of the latter). The augites, indeed, have the 

 appearance of having separated out in a perfectly mobile medium, 

 which cooled very slowly and allowed time for the development of 

 the crystals, and the broad zones around the porphyritic felspars 

 bear witness to the same conditions. 



IV. The Morpeth Dyke. 



This dyke has been investigated by Dr. Teall and analysed by 

 Mr. Stead (see p. 3). 



It resembles very closely in composition those of Collywell and 

 Crookdene. Silica, titania, magnesia, and alkalies are practically the 

 same ; alumina is somewhat higher in the Morpeth Dyke, at the 

 expense chiefly of the iron. It is remarkable in these circum- 

 stances that this dyke should be rich in olivine, while the others 

 are free from it ; and this would seem to point to differences of 

 physical conditions (pressure and temperature ?), rather than to 

 difference of chemical composition, as the determining cause of 

 variation in mineral development. 



In general appearance, it is remarkably like the Tynemouth 

 rock — perhaps a little coarser in texture. The presence of abun- 

 dant olivine, however, as mentioned above, establishes a difference, 

 as this mineral is only represented by some rare and somewhat 

 doubtful serpentinous pseudomorphs in the Tynemouth Dyke. 

 Again, the porphyritic felspars are smaller and less abundant, 

 though similar in appearance and general arrangement to those in 

 the Tynemouth rock. 



There are numerous green patches of irregular shape in the dyke, 

 consisting probably of partly devitrified glass. The same substance 

 occupies the gas-cavities either wholly or partly, thus taking the 

 place of the calcite amygdaloids in the other three dykes. 



Most of the olivines are decomposed, but many fresh specimens 

 occur ; they are colourless and are traversed by deep, dark green 

 grooves roughly at right angles, and by occasional curved cracks. 

 The interference-colours are brilliant. When decomposed, the whole 

 crystal becomes replaced by green pleochroic material, arranged 

 apparently in bundles parallel to the cracks in the original crystal. 

 It is not easy to fix the relative age of the olivine. The fresh 

 pieces never have good outlines, and most of the decomposed pieces 

 present the rounded appearance suggestive of magmatic corrosion. 

 Several sections have been observed included in augite, and therefore, 

 if they belong to the ground-mass generation, they are certainly 

 older than augite. There is very little evidence bearing on the ages 

 of the felspar and the olivine relatively to each other ; the olivines 

 may be porphyritic elements of the same generation as the por- 

 phyritic felspars, but it seems on the whole more probable that 

 they belong to the earliest period of the ground-mass generation. 



