550 Murchisori 's Silurian System. 



less than nine or ten thousand feet — a vast accumulation of 

 strata interposed between the coal measures and Silurian 

 rocks, the importance and extent of which has been little 

 understood until pointed out in the work before us. 



Trap dykes, and other dislocations which were so com- 

 mon in the coal measures, are comparatively rare in the old 

 red sandstone ; but two examples of trap rocks are known 

 throughout the whole extent of the old red sandstone of 

 Herefordshire, Worcestershire, Shropshire, and Monmouth- 

 shire. One of these dykes which is exposed for a length of 120 

 feet and a depth of 50 feet, is in a quarry near Bartestree. 

 The direction of the dyke is WSW. and its width varies 

 from 60 to 20 feet. The prevailing variety of trap is a 

 highly crystalline greenstone, made up of hornblende and 

 felspar, the central masses having more or less of the 

 speroidal, hard, compact structure with the minerals alluded 

 to, finely and intimately mixed. Other portions of the dyke, 

 particularly near the sides, assume a prismatic form, the 

 ends of the prisms being directed towards the walls, and con- 

 tain much felspar, a little quartz, and something like serpen- 

 tine that gives the whole a greasy feel. The dyke passes 

 through the central or cornstone division of the old red sys- 

 tem, and on either side the rocks lie nearly horizontal. The 

 spotted marls and cornstone are converted in contact with 

 the dyke into a purple amygdaloid with kernels and nests of 

 calcareous spar, and in the sandstones the grains of sand 

 are converted into white quartz.* The effect of these 

 changes extends to a distance of several feet into the adjoin- 



* Mr. Murchison refers this effect to the calcareous parts of the 

 cornstone acting as a flux. The aid of a flux may have been neces- 

 sary to change the sandstone into quartz where there was so great 

 a disproportion between the volume of heated matter, and the rocks 

 exposed to its action; but we find the great sandstone formation in India, 

 where it lies in contact with igneous rocks, converted into quartz rock 

 over whole tracts of country, particularly Muflong in the Kasyah hills, 

 the Chittore range on the northern confines of Malwa, Vide report of 

 Coal Committee, p. 26, 79. 



