280 GEOLOGICAL HISTORY OF DOLERITIC ROCKS. 



shows double refraction. It often has a granular aspect, but its 

 texture is due to consolidation during incipient crystallisation. Fel- 

 sitic texture is well seen at Inch Knock, near Coatbridge in Lanark- 

 shire, and in Kaimes Hill and Dalmahoy Hill, near Edinburgh. 



The glassy base is often more or less altered. At first a fine 

 grey dust appears in it, and with further change the glass may be 

 replaced by specular iron, chlorite, calcite, and quartz. Grains of 

 quartz, clear and crystalline, of secondary origin, are common in 

 Scotch dolerites. 



Representatives of Dolerite in Time. Mr. Allport, in 1874, 

 urged that the names of greenstone, melaphyre, and diabase should 

 be discarded, since they are essentially synonyms for basaltic rocks. 

 Melaphyre is only a dolerite of carboniferous age ; and diabase is a 

 dolerite which is so far decomposed that the mineral chlorite is 

 diffused in it. And as the oldest rocks are most altered, it results 

 that most diabases are of Cambrian, Silurian, and Devonian age ; x 

 they are basalts modified by the action of infiltrating water. 

 Hence, accepting these views, we give no detailed account of the 

 rocks so named. 



Modes of Occurrence of Basalt. Basalt forms lava streams 

 which may be traced to their connection with the parent cone in 

 many of the Tertiary volcanoes of the Inner Hebrides, the Auvergne 

 and the Eifel. Interstratified sheets of basalt are characteristic of 

 the British Carboniferous, Devonian, and older Primary rocks. In 

 the form of dykes, however, basalt has a much wider geological 

 horizon, being intrusive in almost every kind of plutonic, meta- 

 morphic, and sedimentary rock, and in all periods of time. 



Contemporary basalt is absent from the secondary rocks in Eng- 

 land, but occurs in Germany, developing prismatic structure in the 

 triassic sandstones which it covers ; while in the Jurassic rocks near 

 Dettingen, basalts are accompanied by massive tuffs and agglomerates. 

 Basalt induces a columnar structure in the Quadersandstein, near 

 Kribitz and Zittau. 



Difference between Basalt and Augite Andesite. The differ- 

 ence between basalt and augite andesite consists essentially in the pre- 

 sence of olivine, for it is only when olivine is present that the rock 

 can be classed as basalt. The percentage of oligoclase may undergo any 

 amount of variation ; andesine and anorthite may both be associated 

 with the labradorite ; and when sanidine occurs it is commonly in 

 twin crystals. 



Tachylyte. The glassy form of basalt called tachylyte is very 

 rare in this country, and has only been described at the edges of 

 basalt dykes, where the rock has a lustrous pitchy aspect, with ex- 

 tremely minute columnar structure and a dark-brown base, full of 

 cumulites which are regarded as the embryos of magnetic crystals, 

 though well-formed crystals of basalt minerals are scattered through 

 the dark-brown glass. Professor Judd has met with this rock at 

 Screpidale in the east of Raasay, at Beal in Skye, at Some north-west 

 1 Allport : Carbon. Dolerites, Q. J. G. S., vol. xxx. 



