76 



MR. LYELL'S THEORY OF THE PARALLEL ROADS. 



exactly the beaches which fringe the Highland 

 lakes of the present day. Those at Coquimbo are 

 greatly wider, and I should think had been caused 

 by the operation of some more violent agent than 

 the others. 



One theory which presents itself to explain 

 these appearances, supposes a lake to have been 

 formed in the valley and to have stood at the 

 level of the highest road, long enough for a flat 

 beach to be produced by stones washed down 

 from above. The water in the lake must next 

 be conceived to have worn away and occasionally 

 to have broken down portions of the barrier 

 across the valley ; this would allow the lake to 

 discharge a part of its waters into the sea, and 

 consequently, to lower its surface to the level of 

 the second road ; and so on successively, till the 

 whole embankment was washed away, and the 

 valley left as we now see it. 



These stones all bear the marks of having come 

 from some distance, and may possibly have been 

 deposited by a river flowing, in ancient times, 

 from the Andes ; whille some vast, though tran- 

 sient cause, may, at one operation, have scooped 

 out the valley, filled it with water, and left a 

 barrier of adequate strength to retain it at the 

 upper level long enough to account for the forma- 

 tion of the beach we now see, which may have 

 been the work of years or of minutes, according 

 to circumstances. By a succession of sudden dis- 

 ruptions of this dam, the supposed lake would be 

 made to stand at different levels ; and the water 

 washing down the sides of the banks would bring 

 along with it the loose stones, gravel, and mud, 

 to the water's edge, where, their velocity being 

 checked, they would be deposited in the form of 

 level beaches. In the Alpine valleys of Savoy, 

 circumstances precisely analogous frequently oc- 

 cur : a great avalanche dams up a stream, and 

 forms a lake which stands at different heights as 

 the barrier of ice successively breaks away, and 

 we can readily trace the different levels at which 

 the water has stood. 



According to the Huttonian theory of the earth, 

 it is supposed that vast masses of solid land have 

 been forced up, from time to time, from the 

 bottom of the sea, with great violence. If this 

 be admitted, it has been suggested that a wave, 

 greater or less in magnitude, according to the 

 size and velocity of the submarine elevation, must 

 inevitably have been produced : and it requires no 

 great effort of the imagination to conceive a wave 

 sufficiently large to submerge the whole of this 

 coast : at least those who have examined the Alps, 

 the Andes, or any other' lofty chain, and have 

 seen the solid strata of rock now elevated on their 

 edges, to the height of many thousand feet, in the 

 air, although bearing indubitable marks of having 

 once been in a horizontal position, and under the 

 sea, will discover nothing extravagant in sup- 

 posing that if they had been thrown up suddenly 

 from the bottom of a deep sea, a huge wave must 

 have been the result. 



P.S.— March, 1840. At the time I wrote the 

 Journals from which these volumes are extracts 

 (in 1H'2.'5) 1 confess J saw no objection, in theory, 

 to the foregoing conclusions. The perusal, how- 

 ever, of Mr. LyelTe admirable book on the " Prin- 

 ciples of Geology/' has quite satisfied me that 



nature does not act per saltum in the manner 

 above conceived, but that all the geological opera- 

 tions of which we have had the means of care- 

 fully examining the traces may be referred to the 

 gradual action of existing causes. 



The following observations, which are taken 

 from Chapter X. of Mr. Lyell's work, will be read 

 with interest by those who are curious in such 

 matters : — 



" As I did not feel satisfied with this explana- 

 tion," (that above given by me,) " I applied 

 to my friend Captain Hall for additional details, 

 and he immediately sent me his original manu- 

 script notes, requesting me to make free use of 

 them. In them I find the following interesting 

 passages, omitted in his printed account : — * The 

 valley is completely open towards the sea ; if the 

 roads, therefore, are the beaches of an ancient 

 lake, it is difficult to imagine a catastrophe suffi- 

 ciently violent to carry away the barrier, which 

 should not at the same time obliterate all traces 

 of the beaches. I find it difficult also to account 

 for the water- worn character of all the stones, for 

 they have the appearance of having travelled over 

 a great distance, being well rounded and dressed. 

 They are in immense quantity too, and much more 

 than one could expect to find on the beach of any 

 lake, and seem more properly to belong to the 

 ocean." 



" I had entertained a strong suspicion," adds 

 Mr. Lyell, " before reading these notes, that the 

 beaches were formed by the waves of the Pacific, 

 and not by the waters of a lake ; in other words, 

 that they bear testimony to the successive rise of 

 the land, not to the repeated fall of the waters of a 

 lake*." 



These parallel roads of Coquimbo have since 

 been visited and carefully examined by Mr. Dar- 

 win, whose Journal on board the Beagle has 

 deservedly excited so much attention in the geo- 

 logical world. At page 423, he has these words : 



" I spent two or three days in examining the 

 step-formed terraces of shingle first described by 

 Captain Basil Hall on the west coast of America. 

 Mr. Lyell concluded from the account that they 

 must have been formed by the sea, during the 

 gradual rising of the land. Such is the case. On 

 some of these steps, which sweep round from 

 within the valley, so as to front the coast, shells 

 of existing species both lie on the surface and are 

 imbedded in a soft calcareous stone. This bed of 

 the most modern tertiary epoch, passes downward 

 into another, containing some living species- asso- 

 ciated with others now lost. Amongst those now 

 lost may be mentioned shells of an enormous perna, 

 and an oyster, and the teeth of a gigantic shark, 

 closely allied to, or identical with, the Carcharius 

 Megalodon of ancient Europe ; the bones of which, 

 or of some cetaceous animal, are also present, in 

 a silicified state, in great numbers. At Guasco," 

 continues Mr. Darwin, " the phenomenon of the 

 parallel terraces is very strikingly seen. No less 

 than seven perfectly level but unequally broad 

 plains, ascending by steps, occur on one or both 

 sides of the valley. So remarkable is the contrast 

 of the successive horizontal lines, corresponding 

 on each side with the irregular outline of the 

 surrounding mountains, that it attracts the atten- 



* Lyell's Principles of Geology, vol. iv. page 111. Fifth 

 Edition. 



