METHOD OF PRESERVING THE- SENSITIVENESS OF COLLODION PLATES. 



[1854. 



mile to an inch, both horizontally and vertically ; for it is only 

 by using the same scale for both measurements that a true idea 

 can be at once conceived of the very small slope in a set of 

 strata that is required to produce important eifects in geograjDhi- 

 cal distribution. 



It will be seen by the section that between the highest forma- 

 tion in the Western District (the Hamilton gToup) and the 

 Carboniferous series, the rocks that are wanting (the Chemung 

 and Portage groups) have a thickness of about 2500 feet, and 

 without a very extensive area of these, there can be no reason- 

 able expectation of coal. 



The position of the great Lakes of the St. Lawrence, and the 

 distribution of the rocks in connection with them, is one of the 

 grandest and most beautiful instances to be met with of the 

 dejjendence of the geographical features of a country upon 

 geological structure. Lake Ontario, Georgian Bay, with its 

 continuance behind the Manitoulin Islands, and Green Bay, in 

 Wisconsin, are excavations in the same formation of the Lower 

 Silurian series. Lake Erie, Lake St. Clair, Lake Huron, and 

 Lake Michigan are excavations in equivalent constituents of 

 the Upper Silurian, while there runs a ridge separating these 

 two sets of excavations from one another, which derives its 

 main characteristic fi'om the Niagara Limestone. The Chemung 

 and Portage groups, which are composed chiefly of sandstones, 

 have been strong enough to resist the denuding forces which 

 have produced the excavations, and we find them forming 

 equivalent limits to the Upper Silurian or perhaps more cor- 

 rectly, Devonian Lakes. It is thus the distribution of these 

 various rocks, which is again dependent in a great measure 

 upon the anticlinal arch running between the two great coal- 

 fields, that gives to a very large part of Upper Canada its present 

 geographical form. 



Let us suppose that there was the smallest possible patch of 

 the Carboniferous series in the Western District. What would 

 be the result? It would be surrounded, of course, by the Che- 

 mung and Portage groups. These would give around the car- 

 boniferous centre a broad ring of sandstone, which would reach 

 as far as Maiden to the south-westward, and London to the 

 north-eastward, and the Western and London Districts, instead 

 of being underlaid chiefly by calcareous, would be so by silieious 

 rocks. The structure in connection with the coal-patch being 

 sinclinal instead of anticlinal, the projected forms of the Cor- 

 niferous limestone would be turned in the opposite directions 

 to those they now have, and in Canada all the formations below 

 would in succession be carried farther to the eastward. With 

 the distribution of the rocks, the forms of the Lakes, dependent 

 on this distribution, would be altered. The sandstones sur- 

 rounding the coal-patch would extend, with the exception of 

 the coal-patch, across from the Michigan to the Appalachian 

 coal-field ; and if like causes are to be supposed productive of 

 like eifects, one-half of Lake Erie and a part of Lake Huron 

 would be obliterated, and the remaining portion modified in 

 form. In short, the supposition of an acre of the trae Carboni- 

 ferous rocks existing in the Western District, requires as a 

 consequence the supposition of a very extensive change in 

 Upper Canadian geography. 



If it be supposed that the coal-patch might be present through 

 the influence of a dislocation, one of the cou,ditions of such a 

 dislocation must necessarily be that it must produce a down- 

 throw on one side or the other of at least 2500 feet ; and it 

 would still be required that on the downthrow side the wide 

 zone of sandstone, and all the circumstances consequent on it, 

 should follow the coal until inten-upted by the fault. But if 



disturbances had occurred in this part of America of suiEcient 

 force to produce a dislocatiorT of this order, it is probable that 

 it would not be a solitary one. The strata of the District would 

 have been tilted up to various high angles, and instead of its 

 flat surface, dependent on the flatness of its rocks, the country 

 would have presented a mountainous one. 



Unless, therefore, workable coal seams are to be found in 

 older rocks than those of the tnie carboniferous age, which no 

 ascertained facts either in the LTnited States or in Canada, or 

 any other part of America, authorize us to expect, it appears 

 to be a necessary consequence of the structure of the Western 

 District that none will be met with there. But though there 

 are no true coal measures in the District, there are rocks which 

 may readily be mistaken for such by observers, who unaware, 

 when actual workable coal seams are not before the eye, how 

 extensive an examination it may be expedient to make, and 

 how many circumstances connected with geological stnicture 

 it may be necessary to bring into harmony, before it is defi- 

 nitely pronounced whether a particular set of strata are Hkely 

 to be associated with coal seams, are disposed to come to a 

 hasty conclusion, founded upon mere mineral resemblances. 

 These rocks are the black bituminous shales of the Hamilton 

 group. They are no doubt nearly identical in mineral character 

 with similar .shales frequently found interstratified with true 

 coal measures. Like them, they in several places hold so much 

 bituminous matter as to give a partially inflamable character to 

 the rock, and to yield petroleum or mineral oU. Not only do 

 they resemble them in mineral character, but also in some 

 degree in respect to a portion of their fossil contents. Coal 

 measures are strongly marked by their fossil plants, and in the 

 Hamilton shales are found Cktlamites; a genus abundant in the 

 Carboniferous rocks, though the species may perhaps be different. 

 These Ckdamites in the Hamilton shales, having lost their inte- 

 rior by decay, are found comp)ressed into flat .stripes and con- 

 verted into crystalline coal, as they generally are under similar 

 conditions in true coal measures. The circumstances of the 

 ease, therefore, might occasionally deceive even jjraetical 

 observers, had they not other guides in the Crustacea and 

 Mollusca of the formation, and a traced out and ascertained 

 place for it in the order of superposition, in which by prior 

 extended examinations its constituent strata had become 

 known. It has been well ascertained by the geologists of the 

 United States, that the place of these shales in Northern New 

 York and Pennsylvania is about 2500 feet beneath the Car- 

 boniferous rocks ; and before the institution of the State geolo- 

 gical surveys, the formation had been very extensively and 

 very expensively examined by boring, excavation, and surface 

 explorations in search of coal seams, but- of course without 

 success ; and it is with a view to aid in preventing a repetition 

 of such useless expenditure in Canada that the present paper 

 and its illustrations are submitted to the Canadian Institute. 



On a method for preserving' the sensitiveness of Collodion 

 Plates for a considerable time. 



By John Spiller and William Ckookes.' 



The extreme sensitiveness of Collodion as compared with 

 paper and other photographic surfaces, renders this material 

 invaluable in all eases where rapidity of. action is desirable, but 



*From the Philosophical Magazine. 



