September 24, 1886.] 



SCIEl^CE, 



271 



SOME FEATURES OF THE RECENT 

 EARTHQUAKE. 



Some knowledge of the local topography and 

 geology in the vicinity of Charleston is essential 

 to a complete understanding of the effects of the 

 great earthquake. 



The seaboard portion of the coastal plain, upon 

 which Charleston, Summerville (twenty-one miles 

 to the north-west), and the other towns most 

 seriously affected by the recent catastrophe, are 

 situated, is a nearly uniform plain from ten to 

 thirty or forty feet in altitude, slightly inclined 

 seaward, and diversified by broad, irregularly 

 meandering, and inosculating troughs and pond- 

 like depressions from five to fifteen feet deep. 

 The depressions, or 'low-grounds' as they are 

 termed colloquially, are frequently swampy, and 

 toward the ocean merge into the tidal channels of 

 the coast ; but, when above the reach of the tide, 

 they are lined with a rich semi-alluvial soil, either 

 supporting luxuriant arboreal vegetation, or pro- 

 ducing abundant crops ; while the uplands con- 

 stituting the plain proper (the ' high-grounds ' or 

 ' pine-barrens ' of the rural population) have a 

 light, sandy soil little charged with humus, and 

 are naturally forested, chiefly with pine. This 

 slightly accented topography is not the product of 

 sub-aerial erosion and deposition, but was fashioned 

 by oceanic waters as the land emerged from the 

 sea ; the high-grounds representing the slightly 

 sloping beaches, and the low- grounds the tidal 

 canals and estuaries, of an epoch during which 

 the land stood from ten to thirty feet lower than 

 now. Summerville is an aggregation of suburban 

 residences irregularly scattered about in a pine- 

 forest on the uplands, and is probably the most 

 elevated point in its longitude between Cooper 

 River on the north-east and the Ashley on the 

 south-west. Ten-Mile Hill (midway between 

 Charleston and Summerville) is on the eastern 

 margin of the same upland, overlooking an irregu- 

 lar depression connecting these rivers ; while 

 Charleston is located on the extremity of a penin- 

 sular prolongation of the plain, bounded on the 

 north-east and south-west respectively by the 

 Cooper and Ashley rivers, which, by reason of the 

 seaward tilting, is elevated but a few feet above 

 tide. 



The geologic structure is remarkably simple, 

 and when the formations have been thoroughly 

 investigated, and definitely correlated with those 

 of other portions of the coastal plain, will doubt- 

 less be found wonderfully uniform over a con- 

 siderable area. The superficial deposit in the up- 

 lands is obscurely stratified, fine yellow sand or 

 (rarely) mottled clay reaching a depth of from 

 five to fifteen feet. Beneath this member, and 



frequently immediately beneath the soil in the 

 low-grounds, occurs a bed of fine clayey sand or 

 silt, generally bluish in color. This stratum com- 

 monly contains sulphurets and various salts, either 

 free or quickly liberated on oxidation. It is from ten 

 to thirty or forty feet thick ; the precise thickness 

 being difficult to determine, partly because of the 

 local thickening due to depressions in the subja- 

 cent surface, and partly because of the impossi- 

 bility of separating it from the superjacent mem- 

 ber : indeed, the superior sand appears to differ 

 from this mainly in the greater amount of oxida- 

 tion which it suffered. In the low-grounds, and 

 along the coast generally, these sands are overlain 

 or replaced by estuarine alluvium consisting of fine 

 blue silt or clay, locally designated ' pluff mud ; ' 

 for the land is now subsiding (and apparently most 

 rapidly south-westward), and sedimentation is ad- 

 vancing upon the land. Beneath these superficial 

 deposits occurs the commonly recognized ' marl- 

 bed,' at the summit of which the South Carolina 

 phosphates are found. The superior strata of this 

 marl-bed in some isolated areas have been referred 

 to the later tertiary by Holmes and others ; but 

 by far the larger portion of the mass represents 

 the formations made classic by Tuomey under the 

 names of ' Ashley and Cooper beds ' and ' Santee 

 marls.' These formations consist of a somewhat 

 variable but nevertheless remarkably uniform 

 succession of marls, clays, and sands, extending 

 to a depth of about six hundred feet where they 

 are underlain by petrographically similar creta- 

 ceous deposits, increasing in heterogeneity some- 

 what downward to two thousand feet below the 

 surface. At this depth a good supply of artesian 

 water has been obtained. The structure at greater 

 depths is not certainly known ; but, according to 

 Hall, the fossils from the lowest strata reached by 

 the artesian borings indicate that a considerable 

 thickness of cretaceous strata are infraposed, 

 while there is reason to believe that these, in turn, 

 rest on pre-cretaceous beds. 



To one traversing the disturbed area, the effects 

 of the earthquake are themselves no more con- 

 spicuous than the indications of inequality and 

 intensity, and variability in character, of the dis- 

 turbance ; and it is this phase of the subject that 

 wiU be dealt with in the following paragraphs. 



1. From the early commencement (Friday, Aug. 

 27) and the long duration (up to date) of the 

 seismic disturbance at Summerville, from the 

 frequent repetition and gi-eat intensity of shocks, 

 from the frequency of detonations and their 

 simultaneity with tremors, and from the verti- 

 cal direction of the vibrations, that place may 

 be regarded as the centre of disturbance. The 

 predominant effects of the shock of Aug. 31 are, 



