GEOLOGY OF TUB ST. CLAIR TUNNKL. 



71 



other parts of Ontario may be of interest for comparison. The determinations were made 

 by Dr. B. J. Harrington and have not hitherto been published : — 



PETROGRAPHICAIi CHARACTER OF THE GkAVEL. 



The gravel in these clays is generally small, passing into saud, but in No. VII. some of 

 it vpas quite coarse, the largest fragments measuring as much as two inches across. It is 

 composed principally of worn fragments of soft brownish-black earthy looking bituminous 

 shale. When struck with a hammer this readily splits up into thin fragments parallel to 

 the lamination, and when held in the flame of a Bunsen burner decrepitates and takes 

 fire, burning for a second or two and giving off a strong tarry odour. 



In almost every case where a broken fragment of the shale is examined by means of 

 a lens it is seen to be thickly strewed with the minute sporocarps of Protosalvinia 

 Huronensis, characteristic of the Chemung (Huron) shales, from exposures of which the 

 fragments were evidently derived. 



In addition to the fragments of Huron shale, a number of fragments, more or less 

 worn, of a soft fine grained, somewhat dolomitic, and micaceous sandstone, are found in 

 the gravel, as well as some much smaller fragments of brownish or yellowish limestone, 

 often highly magnesian. These latter are sometimes pure, and at other times contain a 

 very large amount of siliceous and argillaceous insoluble residue. Occasionally a few 

 rounded fragments of white or greenish quartzite are also found. 



A number of fragments of the sandstone referred to above were crushed and treated 

 with warm dilute hydrochloric acid. A slight effervescence took place, and small amounts 

 of iron, lime and magnesia passed into solution. The dolomite being thus removed, the 

 insoluble residue was mounted and examined with the microscope. A thin section of one 

 fragment was also prepared. The sandstone was found to consist of the following miner- 

 als: — quartz, orthoclase, microcline, plagioclase, muscovite, biotite, hornblende (?), tour- 

 maline, zircon, sphene (?) together with some opaque dark grains, possibly of some carbon- 

 aceous material. In the thin section, which w^as not treated with dilute hydrochloric acid 

 before examination, dolomite and ferric hydrate could also be rcLOgnized. The little crys- 

 tals and grains of tourmaline and sphene (?) closely resembled those which, as mentioned 

 below, were found in the sand occurring with the clay. 



One large well laminated fragment of this sandstone had a structure resembling false 

 bedding and showed what Sir William Dawgon believes to be obscure worm burrows or 

 fucoid markings. 



