46 COLEMAN OlST THE MICEOSCOPIC 



rock, massive rocks giving pebbles with their various diameters not far from equal, while 

 laminated or schistose rocks afford flattened ovoid forms. 



Boulders. — The boulders are of all kinds aud sizes, from larger pebbles to masses of 

 several tous, which the farmer must blast or undermine aud bury to get rid of them from 

 his fields. They are usually more or less rounded, and are very unequally distributed. 

 More than half of the smaller boulders are of Lower Silurian limestone, with the usual 

 fossils, and sometimes nodules of chert, which is not found in Silurian outcrops near 

 Cobourg. 



No further notice of the common limestones of the drift, or of the other clastic rocks, 

 such as slate or sandstone, which are very rare, will be necessary in this paper, which 

 will be devoted to the crystalline rocks. The crystalline limestones will be referred to 

 briefly first, aud afterward the massive and schistose rocks will be treated more at 

 length. 



n. — Crystalline Limestones and Dolomites. 



Macroscopic. — These rocks vary in grain from very coarse to compact, and in color from 

 the pure white of saccharine marble to yellowish and bliiish greys. They shew no traces 

 of fossils, but yet are more or less evidently stratified, aud no doubt are metamorphosed 

 sediments. 



Microscopic. — Under the microscope they are found to consist, of course, of calcite or 

 dolomite, more or less pure, and present the usual rhombohedral cleavage and dull 

 colors with polarised light. As accessory minerals, quartz is the commonest, and may be 

 present to the extent even of one-third, while biotite sometimes occurs in so large an 

 amount as almost to form a calcite schist. In addition, pyrite, muscovite and hornblende 

 are found sparingly. Tremolite and actinolite, or intermediate varieties of hornblende, 

 make up an important part of the rock in a few cases, the tremolite in grey radiating 

 prisms, and the actinolite as handsome sea-green prisms. In one such specimen there are 

 portions of quartz containing fluid cavities and cubes of salt. 



It is perhaps worthy of mention that, in one specimen, the outer layer of calcite proved 

 to be pierced by a network of the rhizoids of lichens. The boring power of these delicate 

 thread-like cells is remarkable, and must depend on the presence of carbonic or some 

 organic acid dissolved in the cell sap. 



III. — Chalcedony. 



A few small masses of chalcedony are found in the drift, derived probably from veins. 

 One specimen is of a pale turquoise blue, fading on exposure, and contains scales of talc 

 and small fragments of other minerals ; while a second specimen is so impregnated with 

 yellow oxide of iron as to approach jasper. 



Microscopic. — Thiu sections display with polarised light the usual microcrystalline 

 structure, varied by radially fibrous parts in which a black cross may be seen. 



