196 



clase, granules of augite, magnetite, and a considerable amount of crypto- 

 crystalline interstitial matter. Near the Manse of Eskdalemuir this rock 

 presents a curious and anomalous structure. The central portion, which is 

 about sixteen or eighteen feet broad, is vitreous, and on either side of this 

 is a band about eight feet thick of the normal rock above described. The 

 line of demarcation between the two portions is perfectly sharp. "The 

 vitreous rock stands up as a wall, on the outer surface of which a curious 

 series of polygonal reticulations may be observed. This external sculptur- 

 ing arises from the internal prismatic structure of the vitreous band. 

 The prisms are of course approximately horizontal, being directed from 

 the nearly vertical walls of the band. Each consists of a central core of 

 vitreous rock, with an external sheath of dull and, to the eye, apparently 

 more crystalline and devitrified rock. It is the union of the sheaths of 

 contiguous prisms which has formed these prominent ribs. Each rib is 

 thus composed of the outer shell of two prisms, the dividing line between 

 which is marked by a suture along the centre of the rib. Moreover 

 each rib is cut into small segments by a system of close joints, which 

 are placed generally at a right angle to the course of the rib. On 

 breaking open the vitreous cores of the prisms, we perceive that 

 the rock, deep iron black in colour, has a peculiar vitreous glaze, quite 

 unlike that of an ordinary basalt ; that here and there the black glass of 

 the base is segregated into kernels, or irregular patches, in which can 

 generally be detected one or more amygdules of chalcedony, but some of 

 which have a black empty internal cavity ; and that throughout the rock, 

 devitrification has proceeded so far as to allow of the individualization 

 of the triclinic felspar (of which distinct facettes may be seen with a lens), 

 and evidently also of other minerals." 



Under the microscope (see Fig. 1, Plate XXIV.), the glassy rock is seen 

 to consist of lath-shaped plagioclase, small grains and crystals of augite, 

 crystals of magnetite and a considerable amount of brown glass, containing 

 in some parts (but not in the part represented in our figure) globulites, 

 trichites, and microlites. The globulites often coalesce to form dart-shaped 

 bodies, which are frequently seen to have clustered round the augite 

 granules in a radial manner. Where this action has taken place, the 

 glass has been bleached. Only 16 '8 p.c. of the rock was found to be 

 soluble after treatment for ten days in hydrochloric acid, with one boiling 

 at the end of the period. 



I. II, 



Si0 2 ... 58-67 ... 65-49 



A1 2 3 ... 14-37 ... 14-66 



Fe 2 3 ... 1-64 



FeO ... 6-94 ... 5-44 



MnO. tr. 



CaO ... 7-39 ... 3-73 



MgO ... 4-65 ... 1-57 



K 2 ... 1-42 ) 



Na 2 ... 3-01 I ... not est. 



H 2 ... 2-02 ) 



100-11 90-89 



I. Analysis of the rock by Mr. WILSON. 

 TI. Partial analysis of the kernels of glass 



