37 



the locally vai)Mng character ami conditions of deposition, and the consequent 

 local ditt'ereticos in tlie life grouping within the areas in which tlie strata, not- 

 witbstandiug their often great lithological diversity', were simultaneously laid 

 dowu either along shore lines or in th^j bordering deeper waters. A formation 

 wliicli in one area is a limestone, in another not far removed, may be entirely 

 formed of san i, or clay, or pebbles, and as a consequence, or from being a shallow 

 or a deep water formation, contain an almost entirely different a^s-eiublage of 

 fossils, and thus neither palteontulogy nor mineralogy, nor liihology, without care- 

 ful and accurate stratigrapbical work, will serve to determine the relative age of 

 formations. All these local changes should, however, bo carefully observed and 

 noted, as they are often both practicallj^and scientifieally important, but they must 

 be very cautiously used in the determination of the age of forma ions. Many amateur 

 geologists go iuto the field, collect a number of specimens of the rock and 

 fossils, some of them perhaps loose, and then on the evidence these aflbrd attempt 

 to determine the geological structure of the region examined. Dips and strikes are 

 not recorded on a map, overlaps are mistaken tor unconformity, and faults or folds 

 causing i-epetition of strata are not observed, and thicknesses in consequence often 

 greatly over-estimated. Again, not unfrequeutly theories are tormed and facts 

 carefully sought to support these, while the pla'n meaning of others adverse to 

 the theories are either ignored or supposed to mean the very reverse of what they 

 indicate. Though in a general way the structure of the Ottawa valley is pretty 

 well known, there are many pala-ontological and structural details which remain to 

 be worked out, and which offer in a comparatively limited and easily accessible 

 area a highly interesting fiuld of investigation, and it is to be regretted that the 

 most essential re(|uisite for the accomplishment of this work, viz., an accurate 

 map on a sufficiently large scale, does not exist. 



One most interesting question is that of overlap or unconformity, and which 

 of these has produced the distribution of the foimations as now observed in the 

 Ottawa Valley. Overlap is frequently met with along the margin of foimations 

 deposited in tracts which were undergoing gradual submersion. As the land 

 sank successive zones would be submerged, and the later deposits on the sea bed 

 "would be prolonged further and further beyond the limits of the earlier ones. 

 This appears to have been the case in the Ottawa valley. But further careful in- 

 vestigation is required to enable us to determine how far the relation of the 

 several formations from the Potsdam to the Hudson Kiver are the result of fault- 

 ing, of unconformity, or of overlap. Several considerable i^iults have already 

 been indicated by Sir \V. Logan. In an overlap the sti'ata are parts of one con- 

 tinuous unbroken series the lormation of which does not appear to have been 

 interrupted by any great physical disturbance, only a quiet and equal subsidence 

 of the land. But where the accumulation of a set of beds or strata has been 

 succeeded by its elevation, exposure, and denudation, the next group of rocks laid 

 down are said to lie unconformably on it. Now, in cases where the older rocks 

 have been equally upraised and again after having been exposed to denudation, 

 ■have been equally depressed, without being folded or tilted, and a newer set of 

 rocks deposited on them, it is often exceedingly difficult to distinguish between 

 this kind of unconformity and an overlap. 



An UQConformity is of the highest importance in the geological structure of 

 a district, because it marks one of the greatest gaps or intervals in its history, and 

 the observer should therefore spare no pains in collecting all available evidence of 

 the existence of such a structure. Sir W. Logan says (in 18G3) the Lower Silurian of 

 Canada may be separated on palreontological grounds into two districts — an upper 

 and a lower, or as 1 propose to call them, a Cambrian and aCambro-Siluriaa, and he 

 puts the line or break at the base of the Black Ri^'er — and, mark, not apparently 

 on stratigrapbical grounds, but on paheontological evidence, which was then im- 

 perfect. S ratigraphically there is no apparent unconformity bet^veen the Chazy 

 .an i Black River, or between it and the Trenton. Later (in 1866) Mr. Billings says, 



