THE ERRATIC 



525 



r 



several consecutive failures of the coffee 

 crop, by unusual rainy seasons, resulting 

 in inundations, destruction of railroads, 

 and depression of the banana trade. 

 A catastrophe like that which has just 



befallen Costa Rica is appalling enough 

 whenever it occurs, but it could not have 

 come at a more unfortunate time in the 

 life of that interesting spot of the 

 American Tropics. 



THE ERRATIC 



By O. a. Ljungstedt, of the U. S. Geological Survey 



WHENE\'ER your vacation 

 rambles have taken you to one 

 of the Northern States of the 

 country, you have no doubt often been 

 struck by the sight of some unusually 

 large boulder perched on the top of a 

 mountain, or resting, maybe, in such a 

 nicely balanced position by the very edge 

 of the sea that a dashing wave may rock 

 it to and fro. 



Examine one of these boulders more 

 closely and you will find several things 

 to distinguish it from others that you 

 may have noticed south of the area 

 show'n as white in the map on page 526. 

 Should the rock on which the boulder 

 lies be bare of soil, it will often be found 

 to be of a different kind than that of 

 which the boulder is composed. Thus 

 the boulder itself may be of granite and 

 rest on a surface of limestone, shale, or 

 sandstone. 



You will also often find this bare rock 

 polished to a remarkable degree, or 

 marred by scratches, and even deep 

 grooves running in a more or less paral- 

 lel direction, known as glacial stri?e. 



To understand how a block of granite 

 as large as a small cabin could be lodged 

 on top of a mountain when there is no 

 similar rock within a hundred miles or 

 more, we must trace its history back to 

 a time at least 200.000 years ago. when 

 the geologic period called the Tertiary 

 was drawing to its close. The first fact 

 to attract our attention, could we have 

 taken a bird's-eye view of the northern 

 part of our continent at that time, would 

 be its greater extent than at present. 

 Looking eastward we would see the 



shoreline extending in places a hundred 

 miles beyond the shoreline of today, and 

 in vain would we scan for the islands, 

 bays, and reaches that now lend such 

 enchantment to our picturesque coast. 



Northward the land probably stretched 

 unbroken over the present Arctic archi- 

 pelago, and connected on the east by way 

 of Greenland, Iceland, and the Scandi- 

 navian Peninsula with Europe and on the 

 west by closing of Bering Strait with Asia. 



This circumstance had a very impor- 

 tant bearing on the fauna of that day, 

 as it enabled the animals from the one 

 continent to cross to the other. [Making 

 a closer inspection of the landscape be- 

 neath us, we would be surprised at the 

 total absence of the smaller lakes that 

 now are its most characteristic feature. 

 Even the Great Lakes were missing, with 

 the possible exception of Lake Superior, 

 while in the valleys in which these latter 

 now lie flowed rivers belonging to one 

 or more systems. 



The mountain groups of today we 

 would recognize at once, notwithstanding 

 their somewhat more rugged outline, and 

 the same would be true in the case of the 

 rivers. While we would see a number 

 of them in strange courses, the master 

 streams we would know at first glance. 

 For millions of years these had b^en cut- 

 ting their channels undisturbed, until at 

 the close of the Tertiary a new impetus 

 was imparted to them, owing to the re- 

 cent rise of land and the ever-increasing 

 humidity of the climate. So we would 

 l^rohably see them tur])ulent and swollen 

 and the sides of their water-courses often 

 precipitous and jagged, overhung in 



