January G, 1905.] 



SCIENCE. 



31 



By weak rocks is meant those which offer a 

 comparatively small degree of resistance to 

 the agencies of abrasion, such as streams, 

 glaciers, etc., or yield readily to the solvent 

 action of water; while resistant rocks are 

 such as have the opposite attributes in these 

 particulars. Certain rocks are weak in refer- 

 ence to both mechanical and chemical erosion ; 

 and of the membei's of this class limestone is 

 by far the most common. 



On account of its comparative softness and 

 solubility, limestone is, as a rule, more easily 

 removed during the process of denudation 

 than the formations with which it is usually 

 associated, and when it occurs at the earth's 

 surface side by side with more resistant rocks, 

 its presence is frequently indicated by a de- 

 pression. So generally is this the case, particu- 

 larly if the rocks referred to occur in essen- 

 tially horizontal beds, that it is a surprise to 

 find limestone forming bold eminences in a 

 region which has been stable for a long time 

 and in which pronounced mechanical and 

 chemical denudation has occurred. Examples 

 of limestone standing in bold relief in regions 

 where, for the most part, these several condi- 

 tions obtain, are furnished by Mackinac 

 Island, situated in the western portion of Lake 

 Huron, and by Gibraltar, the well-known rock- 

 fortress, one of the Pillars of Hercules. 



Mackinac Island has a circumference of 

 about nine miles, and an area of 2,221 acres. 

 It rises to an elevation" of 317 feet above the 

 level of Lake Huron, and the surrounding 

 water within a mile of its shore is from 150 

 to 200 feet deep; its total height above the 

 bottom of the partially submerged valley in 

 which it is situated is thus in excess of 500 

 feet. The rock of which the island is com- 

 posed is limestone, which dips very gently to 

 the south, and at several localities has been 

 eroded so as to form vertical lake-cliffs. Lime- 

 stone belonging to the same geological forma- 

 tion occurs on the neighboring St. Ignace 

 Peninsula, but excepting these two circum- 

 scribed localities, has been deeply denuded 

 over an extensive region, and the depressions 

 formed are now occupied by the waters of 

 Lakes Huron and Michigan. 



Gibraltar rises 1,349 feet above the surface 



of the Mediterranean, and the water within a 

 mile of the borders of the peninsula is from 

 300 to more than 600 feet deep. The length 

 of the promontory is about two and one half 

 miles and its width from 550 to 1,550 yards.* 

 It is composed mainly of limestone in highly 

 inclined strata, and, as is rendered evident 

 from its isolated position and the presence of 

 similar limestone on the African side of the 

 adjacent strait, is a remnant of a once ex- 

 tensive formation. 



Mackinac Island and Gibraltar are similar 

 in several particulars; for example, each one 

 is situated on the border of a navigable strait, 

 and is of great strategic importance, as history 

 has demonstrated; but a more fundamental 

 fact is that they are composed mainly of 

 fissured and cavernous and in part brecciated 

 limestone, which is thus rendered especially 

 favorable for the downward percolation of 

 water. The only conspicuous difference be- 

 tween the two elevations seems to be that 

 the rock of Mackinac Island is essentially 

 horizontal, while the rock of Gibraltar is 

 steeply inclined. In each case bordering 

 precipices are present which, no doubt, have 

 been produced in part by under-cutting by 

 waves and currents, but the isolation of the 

 great rock masses themselves seems to be due 

 to the lowering of the region about them re- 

 spectively, and this lowering, as it seems most 

 reasonable to conclude, has resulted from sub- 

 aerial denudation. Precisely why bold rem- 

 nants of formerly widely extended formations 

 should have been left at these two localities, 

 however, has, so far as I am aware, never been 

 explained. 



Another example of limestone standing in 

 relief in a deeply denuded region, and one 

 which is especially instructive in the above 

 connection, is furnished by a low hill' at 



* A. C. Ramsy, and James Geikie, ' On the 

 Geology of Gibraltar,' in Quarterly Journal of the 

 Geological Society of London, Vol. XXXIV., 1878, 

 pp. 505-541. 



I should, perhaps, state in partial justifica- 

 tion for presenting the present article, that I 

 have visited each of the localities mentioned 

 above, and have at least some first-hand informa- 

 tion concerning them. 



