﻿Yol. 66.] DYKE AT CROOKDEXE (NORTHUMBERLAND). 3 



some interest in so far as this dyke, like the one at Colly well, which 

 can be proved to come to a head, shows peculiarities which are 

 possibly due to this circumstance. 



One of the most interesting features of this dyke is the occurrence 

 in it of patches of felspathic matter. They are best developed in 

 the middle exposure, and are irregular in shape ; in some cases the 

 contact with the basalt is firm and un weathered, and the basalt 

 adjoining the felspar is somewhat finer in texture than elsewhere. 

 In the largest of these aggregates (up to 6 inches in diameter) the 

 junction is weathered, and the surface of the basalt in contact some- 

 what convex. We shall now describe the chemical and petrological 

 characters of the basalt and the felspathic aggregates. 



The Basalt. 



The rock was sampled and crushed, and the portion which passed 

 through a sieve with 30 meshes to the inch, but was stopped by 

 one with 40 meshes, was selected for analysis. It was dried at 

 110° C. ; total iron is reckoned as ferrous oxide (Analysis I). 



To facilitate comparison, the analysis of the Colly well Dyke is 

 included in the following table (II), and also those of the Tyne- 

 mouth and Morpeth Dykes (III & IY). The last two are quoted 

 from Dr. Teall's paper, 1 and have been supplemented by a deter- 

 mination of titania. In the original analysis this would be weighed 

 partly with the silica and partly with the alumina : in the table 

 it has been deducted from the alumina. 



I. II. III. IV. 



SiO, 51-31 5110 5830 51-20 



Al,6, 14-55 16-75 1539 18-89 



TiO, 100 0-96 075 1-14 



FeO" 902 803 4-50 — 



Fe 2 0, — — 4-76 7"57 



MnO 047 037 — — 



CaO 11-61 11-97 10-96 1052 



MeO 6-85 5-89 2-68 6-75 



KO 0-60 0-66 0-94 051 



Na\0 1'79 202 1-74 1-71 



COT 147 1-20 — — 



H 2 (J 1-14 1-16 — 1-70 



Totals ... 99-81 100-11 10002 99-99 



Specific gravity 2-880 2-859 2-842 2-880 



I. Crookclene Dyke. II. Collywell Dyke. III. Tyneinoutk Dyke (analysis 

 by Stead). IV. Morpeth Dyke (analysis by Stead). 



It is a fairly fine and even-grained rock composed of felspar, 

 augite, and iron oxide, with some interstitial matter. The two 

 former make up the mass of the rock, while the iron oxide occurs 

 only in small and imperfect or skeleton crystals. The ground-mass 

 felspars are lath-shaped, and usually show distinct binary twinning; 



1 Quart. Journ. Geol. Soc. vol. xl (1884) pp. 235, 239. 



32 



