168 PROF. T. T. GEOOM 02s^ THE IGNEOUS ROCKS [Feb. I9OI, 



V. Olivine-Basalts. (PI. VII, figs. 2-4.) 



Associated with the Upper Cambrian Shales, and to a much 

 smaller extent with the Holly bush Sandstone,^ are a series of sills 

 and small laccolites which belong to a type different from any 

 of the rocks already described. No example of a dyke crossing the 

 bedding is known to the writer. These rocks are widely distributed 

 in the Shales, which in many instances they have bleached, or 

 indurated, sometimes to a distance of several feet, though in other 

 places the contact-effect has been slight. They differ from the 

 andesitic amphibole-roeks in the nature of the groundmass ; in 

 the presence of phenocrj^sts of olivine (now invariably replaced by 

 serpentine) ; and in the entire absence of phenocrysts of amphibole, 

 augite, and felspar. They vary in thickness from 2 inches to 

 75 feet or more. 



Their relation to the Shales can seldom be determined directly, 

 ■except in the immediate neighbourhood of White-Leaved Oak. In 

 the path leading from this village towards Fowlet Farm thin bands 

 from 1 foot to several yards thick may be seen intercalated between 

 the Black Shales. In the road leading south-westward from the 

 village nearly vertical Black Shales with intercalated thin sills 

 are seen ; one of these sills, 2 feet in thickness, completely thins 

 out in the course of a few yards. The Shales and thin sills are here 

 underlain by a thick igneous mass (M 119). This appears to be a 

 laccolite, for the Shales both above and below are parallel to its 

 surface, and in spite of the considerable thickness (75 or 80 feet) it 

 evidently thins out rapidly south-eastward, as may be inferred 

 from the relief of the surface in that direction and from the absence 

 of any thick sheet of basalt in the lane leading southward from 

 the village : in this lane the succession of Black Shales is interrupted 

 only by two thin sills of olivine-basalt. Thicker basaltic masses 

 evidentl}'" set in again immediately east of the lane. 



At the top of the lane, near the cottage on Chase End Hill, basalts at 

 the base of the 'Middle Igneous Band ' alternate with Grey Shales. 

 One of the former is only 2 inches thick. The ground occupied 

 by the main mass of the igneous band west of the lane rises up 

 into more or less distinct oval elevations, as though concealing a 

 number of inosculating laccolites. Many small lenticular sheets 

 or laccolites are seen near Brousil, where tbey form short wooded 

 ridges. One of these (M 247), which is not less than 20 or 25 feet 

 thick, is seen to be directly overlain by Grey Shales. 



These basalts are of a fairly uniform t3pe, both microscopically 

 and macroscopically, but as the result of weathering they present 

 very different appearances. In their freshest condition (M 117, 

 118, 175, 183, 214, 216, 218, 247, 249, 263, & 268) they are almost 

 invariably compact, dark greyish-blue rocks, with small blackish- 

 green phenocrysts. These latter are usually abundant, but become 

 rare in some cases ; and in others small phenocrysts, only to be 



^ Two of these rocks only (M 117^ & 236) have been cleteeted in it. 



