288 T. W. E. DAVID. 
The structure of the dioritic rocks of that neighbourhood 
described by me at that time as that of laccolites might, I now 
think, be more appropriately termed sill structure. A magnificent 
section illustrative of sill structure is exposed at “ The Falls” 
above the Junction Mine, where a large dyke of diorite may be 
seen to have intersected the claystones almost vertically, and to 
have injected them, parallel or almost parallel to the planes of 
bedding, with sheets of rock from one-eighth of an inch to about 
twenty feet in thickness, and considerably-over one hundred yards 
in length. 
The following passage from my former paper describes the 
Mandurama sills :—“ At first sight the precipitous hill side here 
appears to be composed of alternate beds of eruptive rock and 
altered sedimentary strata, at first mistaken by the author for a 
voleanic series of lavas alternating with fine tuffs. A closer 
examination, however, convinced Mr. Stonier and the author that 
these apparent beds were in reality intrusive laccolites, as 
evidenced by the slightly intrusive character of the junction line 
of their upper and under surfaces with the sedimentaries, their 
unbroken continuity with the diorite of the large dyke, the 
abundance of hornblende in them, and lastly the development of 
small light grey spots in the claystones near the point of contact, 
due probably to the formation of chiastolite. In places the 
laccolites have brought about a partial solution or fusion of the 
intruded sedimentaries, and where they pass into the so-called 
ore beds the author thinks they have intruded and replaced 
probably beds of limestone, absorbing into themselves the lime so 
as to form a type of rock of an ultra-basic character, for which 
perhaps the term Mandwramite may be suggested.” 
The limestone at a neighbouring locality, on Mr. Rothery’s 
Run, as I was informed by the late Professor Stephens, contains 
Pentamerus, and is therefore of Silurian or of Devonian age. 
When the Australasian Association for the Advancement of 
Science met at Hobart in January, 1892, it was the opinion of 
Captain Hutton and some of the other members, including myself, 
