294 PALEOZOIC SYSTEM OF ROCKS. 



elsewhere they are often unconformable. Before taking up the first in 

 the order of time, viz., the Silurian, it is necessary to say something of 

 the interval which in our record separates the Archaean from the Palaeo- 

 zoic era. 



The Interval. 



We have already seen that the lowest Silurian lies unconformably 

 on the upturned and eroded edges of the crumpled strata of the 

 Laurentian. We have also shown (page 179) that unconformability 

 indicates always an oscillation of the earth's crust at the observed 

 place. More definitely it indicates an upheaval, by which the lower 

 series of rocks became land-surface, and were at the same time, per- 

 haps, crumpled ; then a long period unrecorded at that place, during 

 which the land was eroded and the edges of the crumpled rocks were 

 exposed ; then a subsidence, and the deposit of the upper series of rocks 

 on these exposed edges. Now, oscillation necessitates increase and de- 

 crease of land-surface. Evidently, therefore, such increase and decrease 

 of land-surface took place in the unrecorded interval between the 

 Archaean and Palaeozoic eras ; and the length of this unrecorded inter- 

 val is measured by the amount of erosion which the Laurentian under- 

 lying the lowest Palaeozoic has suffered. We have stated that the land 

 at the beginning of the Silurian age was approximately the Laurentian 

 area. The shore-line of the earliest Palaeozoic sea was the line of junc- 

 tion between the Silurian and Laurentian (see map, page 291). But 

 this was not the shore-line at the end of the Archman time. Evidently 

 this shore-line was much farther south ; evidently the land-area was 

 much greater at the end of the Archaean than at the beginning of the 

 Silurian. The Archaean era was closed by the upheaval into land- 

 surface and the crumpling of the strata of the whole Laurentian area, 

 and much more. Then followed an interval of which we know noth- 

 ing, except that it was of long duration, during which the crumpled 

 Laurentian strata forming the then land-surface were deeply eroded. 

 Then, at the end of this interval came a subsidence down to the shore- 

 line already indicated as the Silurian shore-line, and the Silurian age 

 commenced, its first sediments being of course deposited on the exposed, 

 edges of the submerged Laurentian rocks. 



I have attempted to illustrate these facts by the following diagrams 

 (Fig. 267), in which the last, e, represents a north and south section of the 

 Archaean and Palaeozoics of the Canadian border, and southward to Penn- 

 sylvania. The crumpled and eroded strata of the Archaean are seen to 

 underlie unconformably the primordial rocks for some distance south — 

 how far we know not. This is the present condition of things. How 

 they came so is shown in a, #, c, and d. In a, we have the supposed 

 condition of things in Archaean times. The position of the land was 

 somewhere northward — we know not where. In b, a large portion of 



