GEOLOGY AND PETROLOGY OF ENOGGERA GRANITE. 143 



although an occasional dyke is seen which has some points 

 in common with each of these types. The types the author 

 considered in Part I. of this paper under the headings 

 "The Rhyolitic Intrusives" and "The Porphyries." ]\Iore 

 recently Mr. L. C. Ball, B.A., in his report dealing with the 

 geology of the silver-lead deposits near Indooroopilly, 

 included both these groups under the term ' ' Felsites. ' ' 



With his "Notes on Indooroopilly'"^'^ Mr. L. C. Ball, 

 Deputy Chief Government Geologist, publishes a map of 

 the Indooroopilly area on a considerably larger scale than 

 that which accompanies Part I. of this paper, and showing 

 the network of outcrops which the Rhyolitic Series form 

 in this area in much greater detail than was done by the 

 author. The author would like here to digress from the 

 purely petrological point of view to discuss a structural 

 matter of some importance. 



Although Mr. Ball states that ' ' attempts to distinguish 

 laecolites and sills among the dykes are not warranted on 

 the exposures," the author finds in Mr. Ball's descriptions 

 and map several points which appear to uphold his 

 published opinion that the intrusions of the southern area 

 are largely laccolitic, the present outcrops being partial 

 exposures of irregular laecolites. The author never 

 intended to convey the idea that these laecolites were of the 

 ideal type, which rarely has been found in the field. The 

 closest approach to this ideal are the laecolites described 

 by Gilbert from the Henry JMountains, Utah." Gilbert 

 himself, however, figures as an "ideal cross section of a 

 laccolith with accompanying sheets and dykes." a series of 

 intrusions which.* on partial exposure by weathering, might 

 very well give a quite similar outcrop to that in the neigh- 

 bourhood of Indooroopilly. But the types of laccolite most 

 closely approached in the Indooroopilly area are, in the 

 author's opinion, those of the El Late Mountains, Colorado, 

 described by Cross, and the so-called "Cedar-tree" com- 

 pound laccolite described bj^ Holmes from the La Plata 

 Mountains of Colorado. ^'^ A good example of the less 

 regular type of laccolite, and one which the author has had 

 the opportunity of studying in the field, is the Gabbro 



" Qld. Govt. Min, Journal, vol. xxi. p. 266. 



" Eeport on the Geology of the Henry Mts. 1879. 



" Harker, "Natural History of Igneous Rocks," p. 66, fig. 10. 



