116 A POPULAR EXPOSITION OF THE 
and other varieties of hornblende and augite; garnet; tourmaline ; 
condrodite ; spinel; corundum, molybdenite, &c. Descriptions of 
these minerals are given in Parr II. of this series of Papers. A tal- 
cose mineral (Renselaerite or Pyrallolite), probably an altered augite, 
(see Parr II.) occurs also in interstratified beds with the limestones 
of some localities (Ramsay, Grenville, Rawdon); and in Grenville and 
Burgess, yellowish and greenish-grey serpentine occurs under similar 
conditions. Phosphate of lime likewise, occasionally forms irregular 
bands amongst the strata: as, more especially, in North Elmsley, 
South Burgess, and Ross; and at Calumet Falls. 
Some of the limestone beds are of great thickness. According to: 
Sir William Logan, who has devoted much time to an elaborate exe 
amination of the crystalline limestones of the Ottawa region in par- 
ticular, certain beds exhibit a thickness of 1500 feet. In the district 
alluded to, four beds, presenting an aggregate thickness of over 3500 
feet, have been traced out and mapped. For full information respect- 
ing the structural and other characters of these, the reader is referred 
to the Revised Report on the Geology of Canada, by Sir William 
Logan and the other officers of our Geological Survey.* The more im- 
portant localities in which workable beds of crystalline limestone 
occur, will be found under the head of ‘‘ Economic Materials,” below. 
The crystalline dolomites, composed of carbonate of lime and carbonate 
of magnesia, closely resemble the crystalline limestones, and occur un- 
der the same conditions, the two being frequently interstratified. ° A 
fine saccharoidal variety is found on Lake Mazinaw in the County of 
Frontenac, and a more compact kind occurs in the townships of Madoe, 
McNab, Loughboreugh, Sheffield, Grenville, &c. Many of these 
dolomites, it is remarked by Sir William Logan, become of a yellowish 
brown colour by weathering. 
The quartzites and quartzose conglomerates, mentioned above, may 
be referred to in connection with the limestones, as they are generally 
found in their immediate vicinity or otherwise interstratified with them. 
Some beds of quartzite present a thickness of several hundred feet. 
This rock, composed of quartz more or less pure, exhibits a vitreous 
or sub-vitreous aspect, and is either colourless or of a pale reddish, 
brownish, or greenish tint. The quartzose conglomerates are com- 
* To this valuable and truly national work, it may be mentioned here, the present Essay 
is mainly intended to serve as an introduction: illustrating and explaining the various tech-- 
nicalities aud details, a knowledge of which, on the part of the reader, is necessarily presup=. 
posed in the Report in question. 
