BY H. I. JENSEN. 97 



the Glass House Mountains, e.g., Mt. Miketeebumulgrai, Mt. 

 Beerwah, Mt. Ngun-Ngun, and Medway's Mountain, are more 

 porphyritic than the more central members, e.g., Mt. Tunbubudla, 

 Mt. Ewin, Mt. Conowrin, &c, which fact may indicate that the 

 magma partially cooled and consolidated in a deep-seated reservoir 

 in which crystallisation commenced at the borders It seems 

 that the more westerly peaks of the Glass House group are the 

 oldest, for they are the larger, as if most energy had been spent 

 on them, and they are composed of true trachyte (e.g., Mt. 

 Beerwah, Mt. Miketeebumulgrai). When these craters became 

 dormant, points of eruption formed east of them on definite 

 fissure-lines, to again yield place to others more and more to the 

 east. The main axis of the group runs from S S.E to N.N.W., 

 through the Round Mountain, Bridge's Hill (i), Miketeebumul- 

 grai, Tunbubudla and Beerwah. Crossing this axis almost at 

 right angles are various lines of fissure on which the volcanic 

 foci are situated. This arrangement of the mountains has a 

 striking resemblance to that of the volcanoes surrounding 

 Fonseca Bay in Central America. Here the main fissure runs 

 N.W.-S.E., and is crossed at right angles by other fissures on 

 which rows of volcanoes are situated, the most westerly of which 

 is always active, the others successively less and less so as we 

 advance in an eastward direction, when finally the most easterly 

 extinct ones are reached.* If the analogy is true the Glass 

 House Mountains lay to the west of a sea or bay. 



The arrangement of the columns of the prismatic comendites 

 as at Mt. Conowrin, Mt. Tibrogargan, Mt. Coolum and Mt. 

 Cooran (Text fig. 17) is strongly in favour of the mamelon origin of 

 these heights. They may, however, be plugs injected into now 

 denuded tuff cones; or some may have arisen like the lava plug 

 of Mt. Peleef and similar plugs on the Deccan.J 



* See Suess, "La Face de la Terre," Tome i. ' Exemples des Regions. 

 Ebranl^es.' 



tSee " Nature," Oct. 1st, 1903. 

 I See Richard Straehey, in " Nature," Oct. 15th, 1903. 



7 



