398 



ME. F. RUTLEY ON ERUPTIVE ROCKS FROM THE 



That these irregular patches and labyrinths of angite represent 

 individual crystals and not aggregates of crystals, there can be no 

 doubt whatever, since extinction takes place simultaneously and 

 uniformly, both over the connected and the isolated portions of each 

 patch or labyrinth. 



JFig. 2. — Augite-anclesite* from Oarlion, near St. Minver, Cornwall, 



The figure shows one of the remarkably labyrinthine crystals of augite as it 

 appears in a position of maximum extinction between crossed nicols. Part 

 of another crystal of augite, lying in a different azimuth, is represented in 

 the lower left-hand corner of the drawing. The interstitial portions 

 consist of altered felspars and felsitic matter. 

 Specimen no. 7, X 25. Crossed nicols. 



Here, then, somewhat as in the case of micropegmatite, we have 

 a well-marked instance of interruption in the development of indi- 

 vidual crystals by the synchronous formation of smaller crystals of 

 another mineral f. 



Similar interruption in the development of crystals is, indeed, a 

 very common feature in crystalline eruptive rocks ; but it is seldom 

 seen to occur in so striking a manner as in this rock from Carlion. 

 Different grades of such interrupted crystallization are shown in Prof. 

 Judd's recent paper on the " Gabbros, Dolerites, and Basalts of Ter- 



* In using this term, mineral constitution only has been taken into con- 

 sideration.— R E., June 28, 1886. 



t In this case, however, the mineral causing the interruption consists of small 

 individual crystals, thus differing from micro-pegmatitic structure, which is due 

 to interruption of crystallization in both of the minerals concerned. 



